Add cstring unittests and fix cstring<N>::back

[joro75]

• Add unittests for cstring and cstring<0>
• Fix cstring::back() to return last instead of '\0'
• Fixes back always returns '\0' #71

The cstring<N>::back() previously returned chars_[N] which is the terminating '\0', while it should return chars_[N - 1].