library/string.m:
Define string.to_char_list and string.to_rev_char_list to either
replace code units in ill-formed sequences with U+FFFD or return
unpaired surrogate code points.
Use Mercury version of do_to_char_list instead of updating
the foreign language implementations.
tests/hard_coded/Mmakefile:
tests/hard_coded/string_char_list_ilseq.exp:
tests/hard_coded/string_char_list_ilseq.exp2:
tests/hard_coded/string_char_list_ilseq.m:
Add test case.
