Fix qr'\N{U+41}' on EBCDIC platforms

Fix qr'\N{U+41}' on EBCDIC platforms

Prior to this commit, the regex compiler was relying on the lexer to do
the translation from Unicode to native for \N{...} constructs, where it
was simpler to do.  However, when the pattern is a single-quoted string,
it is passed unchanged to the regex compiler, and did not work.  Fixing
it required some refactoring, though it led to a clean API in a static
function.

This was spotted by Father Chrysostomos.