Use a single websocket connection per browser, removing the need for an
extra websocket connection per target.
This is thanks to the Target.sendMessageToTarget command to send
messages to each target, and the Target.receivedMessageFromTarget event
to receive messages back.
The browser handles activity via a single worker goroutine, and the same
technique is used for each target. This means that commands and events
are dealt with in order, and we can do away with some complexity like
mutexes and extra go statements.
First, we want all of the functionality in a single package; this means
collapsing whatever is useful into the root chromedp package.
The runner package is being replaced by the Allocator interface, with a
default implementation which starts browser processes.
The client package doesn't really have a place in the new design. The
context, allocator, and browser types will handle the connection with
each browser.
Finally, the new API is context-based, hence the addition of context.go.
The tests have been modified to build and run against the new API.
