Add alias type definitions for Java

When a spec defines a Model at the top level that is a non-aggretate type (such
as string, number or boolean), it essentially represents an alias for the simple
type. For example, the following spec snippet creates an alias of the boolean
type that for all intents and purposes acts just like a regular boolean.

    definitions:
      JustABoolean:
        type: boolean

This can be modeled in some languages through built-in mechanisms, such as
typedefs in C++. Java, however, just not have a clean way of representing this.

This change introduces an internal mechanism for representing aliases. It
maintains a map in DefaultCodegen that tracks these types of definitions, and
wherever it sees the "JustABoolean" type in the spec, it generates code that
uses the built-in "Boolean" instead.

This functionality currenlty only applies to Java, but could be extended to
other languages later.

The change adds a few examples of this to the fake endpoint spec for testing,
which means all of the samples change as well.

samples/client/petstore/java/okhttp-gson/docs/FakeApi.md

@@ -4,11 +4,195 @@ All URIs are relative to *http://petstore.swagger.io:80/v2*
 
 Method | HTTP request | Description
 ------------- | ------------- | -------------
+[**fakeOuterBooleanSerialize**](FakeApi.md#fakeOuterBooleanSerialize) | **POST** /fake/outer/boolean | 
+[**fakeOuterCompositeSerialize**](FakeApi.md#fakeOuterCompositeSerialize) | **POST** /fake/outer/composite | 
+[**fakeOuterNumberSerialize**](FakeApi.md#fakeOuterNumberSerialize) | **POST** /fake/outer/number | 
+[**fakeOuterStringSerialize**](FakeApi.md#fakeOuterStringSerialize) | **POST** /fake/outer/string | 
 [**testClientModel**](FakeApi.md#testClientModel) | **PATCH** /fake | To test \"client\" model
 [**testEndpointParameters**](FakeApi.md#testEndpointParameters) | **POST** /fake | Fake endpoint for testing various parameters 假端點 偽のエンドポイント 가짜 엔드 포인트 
 [**testEnumParameters**](FakeApi.md#testEnumParameters) | **GET** /fake | To test enum parameters
 
 
+<a name="fakeOuterBooleanSerialize"></a>
+# **fakeOuterBooleanSerialize**
+> Boolean fakeOuterBooleanSerialize(body)
+
+
+
+Test serialization of outer boolean types
+
+### Example
+```java
+// Import classes:
+//import io.swagger.client.ApiException;
+//import io.swagger.client.api.FakeApi;
+
+
+FakeApi apiInstance = new FakeApi();
+Boolean body = true; // Boolean | Input boolean as post body
+try {
+    Boolean result = apiInstance.fakeOuterBooleanSerialize(body);
+    System.out.println(result);
+} catch (ApiException e) {
+    System.err.println("Exception when calling FakeApi#fakeOuterBooleanSerialize");
+    e.printStackTrace();
+}
+```
+
+### Parameters
+
+Name | Type | Description  | Notes
+------------- | ------------- | ------------- | -------------
+ **body** | [**Boolean**](Boolean.md)| Input boolean as post body | [optional]
+
+### Return type
+
+**Boolean**
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: Not defined
+ - **Accept**: Not defined
+
+<a name="fakeOuterCompositeSerialize"></a>
+# **fakeOuterCompositeSerialize**
+> OuterComposite fakeOuterCompositeSerialize(body)
+
+
+
+Test serialization of object with outer number type
+
+### Example
+```java
+// Import classes:
+//import io.swagger.client.ApiException;
+//import io.swagger.client.api.FakeApi;
+
+
+FakeApi apiInstance = new FakeApi();
+OuterComposite body = new OuterComposite(); // OuterComposite | Input composite as post body
+try {
+    OuterComposite result = apiInstance.fakeOuterCompositeSerialize(body);
+    System.out.println(result);
+} catch (ApiException e) {
+    System.err.println("Exception when calling FakeApi#fakeOuterCompositeSerialize");
+    e.printStackTrace();
+}
+```
+
+### Parameters
+
+Name | Type | Description  | Notes
+------------- | ------------- | ------------- | -------------
+ **body** | [**OuterComposite**](OuterComposite.md)| Input composite as post body | [optional]
+
+### Return type
+
+[**OuterComposite**](OuterComposite.md)
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: Not defined
+ - **Accept**: Not defined
+
+<a name="fakeOuterNumberSerialize"></a>
+# **fakeOuterNumberSerialize**
+> BigDecimal fakeOuterNumberSerialize(body)
+
+
+
+Test serialization of outer number types
+
+### Example
+```java
+// Import classes:
+//import io.swagger.client.ApiException;
+//import io.swagger.client.api.FakeApi;
+
+
+FakeApi apiInstance = new FakeApi();
+BigDecimal body = new BigDecimal(); // BigDecimal | Input number as post body
+try {
+    BigDecimal result = apiInstance.fakeOuterNumberSerialize(body);
+    System.out.println(result);
+} catch (ApiException e) {
+    System.err.println("Exception when calling FakeApi#fakeOuterNumberSerialize");
+    e.printStackTrace();
+}
+```
+
+### Parameters
+
+Name | Type | Description  | Notes
+------------- | ------------- | ------------- | -------------
+ **body** | [**BigDecimal**](BigDecimal.md)| Input number as post body | [optional]
+
+### Return type
+
+[**BigDecimal**](BigDecimal.md)
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: Not defined
+ - **Accept**: Not defined
+
+<a name="fakeOuterStringSerialize"></a>
+# **fakeOuterStringSerialize**
+> String fakeOuterStringSerialize(body)
+
+
+
+Test serialization of outer string types
+
+### Example
+```java
+// Import classes:
+//import io.swagger.client.ApiException;
+//import io.swagger.client.api.FakeApi;
+
+
+FakeApi apiInstance = new FakeApi();
+String body = "body_example"; // String | Input string as post body
+try {
+    String result = apiInstance.fakeOuterStringSerialize(body);
+    System.out.println(result);
+} catch (ApiException e) {
+    System.err.println("Exception when calling FakeApi#fakeOuterStringSerialize");
+    e.printStackTrace();
+}
+```
+
+### Parameters
+
+Name | Type | Description  | Notes
+------------- | ------------- | ------------- | -------------
+ **body** | [**String**](String.md)| Input string as post body | [optional]
+
+### Return type
+
+**String**
+
+### Authorization
+
+No authorization required
+
+### HTTP request headers
+
+ - **Content-Type**: Not defined
+ - **Accept**: Not defined
+
 <a name="testClientModel"></a>
 # **testClientModel**
 > Client testClientModel(body)