library/string.m:
Simplify string.append(out, out, in) and make it work sensibly in
the presence of ill-formed code unit sequences, breaking the input
string after each code point or code unit in an ill-formed sequence.
tests/hard_coded/Mmakefile:
tests/hard_coded/string_append_ooi_ilseq.exp:
tests/hard_coded/string_append_ooi_ilseq.exp2:
tests/hard_coded/string_append_ooi_ilseq.m:
Add test case.
