Removed second version of copy_string_to_sstring because it didn't work.
Added a new version of append_n_sstring.
Fixed the tests to use proper arguments.
Forgot to make a standard C test.
Fixed a typo.
Removed the slower version of append_n_sstring.
Created functions find_char(s)_in_sstring and made tests for them.

They are intended to replace s_str(r)chr, s_str(c)spn and s_strpbrk as they are rather unreadable and slow.
Made i in find_char_in_sstring unsigned so the comparison is between 2 unsigned variables.
Fixed the capitalization of false.
Added stdbool.h
Removed unnecessary header - stdint.h
Added another define to make bool look nicer when defined.
Removed the obsolete functions for finding char(s) in a sstring.

They were slow and their names unreadable, not to mention there was a lot of overlap in their use.
Removed their tests as well.
Added documentation for find_char(s)_in_sstring.