Remove long_u: do_outofmem_msg().

Remove long_u occurrences due to do_outofmem_msg() function.
Refactor size parameter from long_u into size_t.
Remove long_u: hashtab: Cleanup: Comments.

- Restyle comments (/// when appropiate, // otherwise).
- Improve comments (add new comments, augment/clarify existing ones).
Remove long_u: hashtab: Cleanup: Others.

hashtab.h:
- Add missing includes.
- Move hash_T to the top and use it to define hashtab_T.
- Move hash_removed related definitions to the top, as they are used in
  the definition of hashtab_T.
- Reformat multiline expression (start continuation with operator).
- Reformat function declaration into one single line.

hashtab.c:
- Use C99 style variable declarations (move declarations as near to
  first-usage point as possible).
- Simplify else branch.
- Remove redundant casts.
Remove long_u: hashtab: Enable -Wconversion.

- Add hashtab.c to converted files list.
- Fix conversion issues:
    * hash_lookup()     : idx      : unsigned -> hash_T.
    * hash_may_resize() : minitems : int      -> size_t.
    * hash_may_resize() : newi     : unsigned -> hash_T.
    * hash_may_resize() : minsize  : long_u   -> size_t.
Remove long_u: hashtab: Refactor long_u type.

hashtab.h:
- hash_T: long_u -> size_t.

  In principle, a hash value could thought of as just an unsigned number
  without size semantics (uint32_t or uint64_t).  But it is used as
  index at some places, and so, size_t is also eligible. Therea re some
  places where assignments occur between hash_T and size_t variables, in
  both directions. Therefore, if we define hash_T to be of a type having
  a different width than that of size_t, we will have an incorrect
  assignment somewhere that will require an assert/guard.  So the most
  sensible option here seems to do hast_T to be size_t too.

- hashtab_T.ht_mask: long_u -> hash_T.

  Masks are used to be combined with hash_T values, so they should be of
  the same type.

hashtab.c:
- hash_may_resize(): oldsize: long_u -> size_t.
- hash_may_resize(): newsize: long_u -> size_t.
- hash_may_resize(): newmask: long_u -> hash_T.
Remove long_u: hashtab: Refactor other types.

Current type of some other parameters/variables can be improved:
- hashtab_T         : ht_error : int  -> bool.
- hash_clear_all()  : off      : int  -> unsigned int.
- hash_clear_all()  : todo     : long -> size_t.
- hash_may_resize() : todo     : int  -> size_t.
Remove long_u: hashtab: Enable clint: Sort clint file.
Remove long_u: hashtab: Enable clint: Add to clint.

- Add hashtab.[ch] to clint file.
- Fix clint errors.