Merge pull request #8 from DuffsDevice/make_public_get_num_bytes

Made useful helper methods public

Merge pull request #6 from DuffsDevice/fix_swap

Fixed swap

Hopefully last fix of test.cpp to make compilation with clang over Tr…

…avis CI work

Finally applied Testing to the most complex function: utf8_string::re…

…place! Fixed many many bugs in it.

Fixed bug in compute_multibyte_table() with codepoints spanning exact…

…ly 2 bytes

Optimized get_num_resulting_bytes()
Removed unnecessary cast to uint8_t in get_indices()