[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.
I do agree that using fixed-sized types in our own code for the most time possible would be clearer.
But that doesn't solve the problem. It only delays it until the last moment. Because, in the end, you have to call a posix-defined function using `off_t` (`lseek` in this case), which is defined that way, and you cannot change it.
So, you still don't have a way to check the invocation to `lseek` is in-bounds.
This particular instance of the problem is a good example.
We are defining variable `offset` just before calling `lseek`.
And all I want is checking the param to `lseek` is in bounds.