library/string.m:
Define behaviour of string.replace_all on ill-formed code unit
sequences when the pattern is empty.
Implement that behaviour.
Use better variable names in documentation of string.replace and
string.replace_all.
tests/general/string_replace.exp:
tests/general/string_replace.exp2:
tests/general/string_replace.m:
Extend test case.
Update code style.
