[RDY] Remove project-specific integer types: long_u. (2)

This is part of a series of PR's aimed at removing long_u from the codebase.
This PR deals with the following instances of long_u:

- Those within *eval* module.
- Those within *memfile* module.

It continues the work in #656, #757 towards fulfilling #459.
Can `ptrdiff_t` (C99) be used instead of `off_t` (POSIX)? Old discussion: https://github.com/neovim/neovim/issues/682#issuecomment-44571076