regcomp.c: Add code to compute edit distance (Damerau–Levenshtein)

regcomp.c: Add code to compute edit distance (Damerau–Levenshtein)

This will be used in a future commit.

This code is taken from CPAN Text::Levenshtein::Damerau::XS with the
author's knowledge.  There have been white-space changes to make it
conform better to perl's core coding standards, and declaration changes
to make it more portable, such as using UV instead of 'unsigned int',
and PERL_STATIC_INLINE instead of a less portable form, but the logic is
unchanged.  One variable was changed to signed from unsigned to avoid a
warning message from some compilers.

The author and I will decide later about keeping the cpan module and
this code in sync.  It changes very rarely.