library/string.m:
Define first_char/3 to fail if the input string begins with an
ill-formed code unit sequence.
Define the reverse mode to throw an exception on an attempt to
encode a null character or surrogate code point in the output
string.
Reimplement first_char/3 in Mercury.
hard_coded/Mmakefile:
hard_coded/string_first_char_ilseq.exp:
hard_coded/string_first_char_ilseq.m:
Add test case.
