library/string.m:
Define behaviour of char_to_string when the string is not
well-formed or if the char is a surrogate code point.
Implement char_to_string/2 using multiple clauses
as the described behaviour doesn't match to_char_list/2.
tests/hard_coded/Mmakefile:
tests/hard_coded/char_to_string.exp:
tests/hard_coded/char_to_string.exp2:
tests/hard_coded/char_to_string.m:
Add test case.
