Remove FIXME comment about disambiguating named subpattern references.

This question has been answered an implemented in a previous commit.
Change FIXME comment in COMPUTE-OFFSETS method on SUBPATTERN-REFERENCE.
Remove another FIXME comment.

As with their offsets, determing the minimum lengths subpattern
references is only feasible for patterns that don't really need
subpattern references to begin with.
Remove the FIXME comment from the REGEX-LENGTH method for SUBPATTERN-REFERENCE.
Remove unused sub from perltest.pl eval.
Make perltest.pl handle arbitrarily large and variable numbers of registers.

The perltestdata file produced by this updated perltest.pl only
reports results for registers that are actually contained in the
corresponding pattern.
Add two tests for double- and triple-digit register number subpattern references.