Simplify usage of Gson from Feign.Builder

The logic in GsonCodec was split into GsonEncoder and GsonDecoder, each of which can
now be used separately.  GsonCodec was deprecated, and can be removed in the next major
version.  To facilitate use outside of Dagger, the double-to-int map type adapter was broken into
its own class, and is included by default when using the default constructors of either the
encoder or decoder.  The examples have been updated to use the new encoder/decoder instead
of the codec.