Examine performance overhead of Character.digit for common radixes

While examining the patch for JDK-8196334 it was made apparent that calling into Character.digit(ch, 16) does not eliminate all branches, which meant that specializing the lookup of hexadecimal in UUID was still profitable.

Investigate if there's something we can do to optimize Character.digit(ch, 16) (and likely digit(ch, 10)) and consolidate logic that specialize for certain radixes - such as in UUID.fromString