Bugfix: Incorrectly performing nulless string appends and prepends.

Bugfix: Incorrectly performing nulless string appends and prepends.

These problems are exposed by the tests that I am writing.

The destination->used < length check cannot be performed because of the presence of NULL characters.
The nulless versions of the strings may be identical even if their sizes do not match.
The only case where this check should still happen is when destination.used is 0.

The comparison operator is using the wrong variable in some cases ('j' should be used with 'destination').

Replace uint8_t with f_char_t.

Minor updates to the documentation comments.