t/test.pl: Allow native/latin1 string conversions to work on utf8.

t/test.pl: Allow native/latin1 string conversions to work on utf8.

These functions no longer have the hard-coded definitions in them,
but now end up resolving to internal functions, so that new encodings
could be added and these would automatically understand them.

Instead of using tr///, these now go character by character and
converting to/from ord, which is slower, but allows them to operate on
utf8 strings.

Peephole optimization should make these essentially no-ops on ascii
platforms.