[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 you indent the bullets:

```
+/// - Blocks numbered from 0 upwards have been assigned a place in the actual
+///   file. The block number is equal to the page number in the file.
+/// - Blocks with negative numbers are currently in memory only. They can be
+///   assigned a place in the file when too much memory is being used. At that
+///   moment, they get a new, positive, number. A list is used for translation of
+///   negative to positive numbers.
```