Define a "Common" header for importing Macroni's dependencies

[PappasBrent]

Currently, Macroni uses types defined by its dependencies directly, e.g., vast::mcontext_t, clang::Stmt; however this makes Macroni more difficult to maintain in the long run because if one these dependency-defined types changes, we need to update all parts of Macroni that use the type. By aliasing all dependency-defined types in one header with using statements, then we make it so that if one of these dependency-provided types changes, we only have to change its definition in this common header.

This is an easy change but a little tedious, and will touch many different parts of Macroni's codebase, so we should address it in its own PR.