Added bs_save_hex method and unit test.

Updated:
 - include/bs.h to add function signature
 - src/bs_hex.c for code
 - tests/bs_hex.c for unit test
Fix to bs_string test to free memory after use.
Changed bs_byte_get and bs_byte_set to bs_get_byte and bs_set_byte.

These names were confusing and felt backwards.
Changed everywhere they're used:
 - include/bs.h - external API
 - a few source files which are using these - need to change this
 - src/bs.c - implementation
 - tests/* - all test files use these to access internal data