Add the predicates:
string.to_utf8_code_unit_list/2
string.to_utf16_code_unit_list/2
string.from_utf8_code_unit_list/2
string.from_utf16_code_unit_list/2
library/string.m:
Implement the above predicates in Mercury code that works across
backends.
Add internal_encoding_is_utf8 helper predicate.
Forward the new predicates to string.to_code_unit_list and
string.from_code_unit_list when the named encoding matches that
of the backend in use.
tests/hard_coded/Mmakefile
tests/hard_coded/string_code_unit.exp
tests/hard_coded/string_code_unit.m
Add test case.
NEWS:
Announce the additions.
