This PR factors out the code that knows how to walk just the live parts of the AST.
The traverser in reflect.cpp is renamed to TReflectionTraverser, and inherits from
TLiveTraverser, which will also be used by a future binding offset PR.
The code is now smart about the entry point name (no longer hardcoded to "main").
There is an option to traverse all code (live+dead), because a consumer of the
class may wish to use it for both purposes without wanting a whole separate
class hierarchy.
