[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.
This is exactly why I advocated for `(u)int_64` to be used for these things. It's quite future proof (max capacity is will be unattainable for probably hundreds of years) and it's readily cross-platform. Libuv does the same thing.