[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.
Ok. Then I'll remove the full comment, as it doesn't have sense to explain why are we doing that anymore. I'll move the last part of the comment to the assignment above this. I mean, it will result in:

```
  // fname cannot be NameBuff, because it must have been allocated.
  mfp->mf_fname = fname;
  mf_set_ffname(mfp);
```