Stubify broadly takes either tbd files or binary dylibs and turns them
into tbd files. In future patches, stubify will also allow additional
information to be embedded into the final TBD output too.
Add Util APIs to TextAPI for common operations used by readtapi for now.
Merge allows a user to merge different files (tbds for now or dylibs in
the future) to emit out a single tbd with all input contents. This does
require that all inputs represent the same library.