This can simplify some common use cases, like running a few actions
directly, or running no actions at all. It's also an almost entirely
backwards compatible change, as all Run call sites should continue to
compile and work.
Leave Tasks, as it can still be useful for functions to return complex
sequences of actions as a single Action.
First, collapse Browser.Start with NewBrowser. There's no reason to
split them up.
Second, unexport Browser.userDataDir, since it's only needed for a test.
It's also a bad precedent, as only the ExecAllocator will control the
user data directory.
Third, export Context.Browser, since we were already exporting
Context.Allocator.
Finally, remove the Executor interface, a duplicate of cdp.Executor.
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.
