Expose headers

Expose header functions (everything from ghc/includes). Mostly for documentation, since the recommended way to use them is still FFI imports.

Probably worth putting in its own module for this reason

Start by reviewing the functions under ghc/includes and the current FFI import approach described in the issue. Determine how they could be exposed through a separate module, and consider the work complete when the intended header functions are available there with documentation covering their use.

Written by the indexing model from the issue text.