Move frog/leg to lib/compiler/implementation.

frog/leg/README.txt

-Leg is an experimental add-on to the Frog compiler that aims to
-explore areas of long-term interest:
-
-   * high-performance extensible scanner and parser
-   * concrete type inferencing
-   * fancy language tool support
-   * programming environment integration
-   * SSA-based intermediate representation
-   * adaptive compilation on the client
-   * ...
-
-Some of the things listed above are very experimental of nature and
-may prove themselves infeasible or unnecesary, so we want to work on
-them in a way that guarantees we will not disrupt the work being done
-to make Frog a fantastic compiler for Dart in its default setup.
-
-To keep things simple and allow quick experimentation, Leg will only
-support a subset of Dart and fall back on using the default Frog
-compiler for the remaining parts of the language. We expect Leg to be
-a complete and correct implementation of the supported subset
-throughout the implementation and experimentation so you should always
-be able to try Leg on any Dart project.
-
-The plan is to share code between the default Frog compiler and Leg
-where it makes sense and to learn from both code bases along the way.