library/string.m:
Define how string.codepoint_offset counts code units in ill-formed
sequences.
Delete C and C# foreign implementations in favour of the Mercury
implementation that has the intended behaviour.
(The Java implementation uses String.offsetByCodePoints which
also matches our intended behaviour.)
tests/hard_coded/Mmakefile:
tests/hard_coded/string_codepoint_offset_ilseq.exp2:
tests/hard_coded/string_codepoint_offset_ilseq.m:
Add test case.
