For future profiling work to eliminate/speed-up slow tests.

This makes grpc.early_adopter much more independent of RPC
Framework and cleaner at the cost of reexporting most of the
interfaces and writing several delegation classes.

Allows run_tests to focus on one particular test if necessary.
Useful in combination with runs_per_test.

We have many assumptions about languages baked into the test system, and we want this test harness to trigger when testing C++ stuff, so it needs to be written in C++.

This provides for now what should be a nicer interface for code
generation than that of the Face layer. In terms of abstraction
it's conceptually very similar so the two should probably be
merged as soon as is reasonable.

I'm honestly a little shocked this wasn't done previously.

A top-level "grpc" package is added and RPC Framework
is made public.

This allows caching results with --runs_per_test.

Otherwise the tests just stop once they succeed, or don't even run at
all if they passed in a previous run.