Add macro for converting Latin1 to UTF-8, and use it

This adds a macro that converts a code point in the ASCII 128-255 range
to UTF-8, and changes existing code to use it when the range is known to
be restricted to this one, rather than the previous macro which accepted
a wider range (any code point representable by 2 bytes), but had an
extra test on EBCDIC platforms, hence was larger than necessary and
slightly slower.