Optimiser: Correctly deal with strings starting with unicode characters in exprConApp_maybe

For example:

"\0" is encoded to "C0 80", then the rule would correct use a decoding
function to work out the first character was "C0 80" but then just used
BS.tail so the rest of the string was "80". This resulted in

"\0" being transformed into '\C0\80' : unpackCStringUTF8# "80"

Which is obviously bogus.

I rewrote the function to call utf8UnconsByteString directly and avoid
the roundtrip through Faststring so now the head/tail is computed by the
same call.

Fixes #19976