And add tests! This implementation closely follows the current C++
implementation, replacing macros and gotos with a slightly more
complex state machine. It's very possible that an async version that
yields tokens on "emit" would be even simpler, but let's get this
one working first :).
At the same time, simplify CMakeLists magic for libraries that want to
include Swift code in the library. The Lib-less name of the library is
now always the module name for the library with any Swift additions,
extensions, etc. All vfs overlays now live in a common location to make
finding them easier from CMake functions. A new pattern is needed for
the Lib-less modules to re-export their Cxx counterparts.
