Changed the printed text to follow what's being tested.
Removed second append_sstring().
Removed unnecesssary documentation.
Changed more function's return values to int and made the return on success more reasonable.

Returning something that is easily known (lenght of the new string) is pointless so no need to do that.
Removed silly third version of append_n_sstring.
Made append use a faster version.
Made append_sstring faster.
Decreesed the number of loops.

The tests seem to give bad numbers when it takes too long.
Simplified time_differenc.
Made i and j long so that they are unlikely to overflow.
Hopefully fixed the problem where the first loop didn't matter.
Changed the tests to append_n_sstring now.