idea is that we perform multibyte->wide character conversion while parsing
and compiling, then convert byte sequences to wide characters when they're
needed for comparison and stepping through the string during execution.
As with tr(1), the main complication is to efficiently represent sets of
characters in bracket expressions. The old bitmap representation is replaced
by a bitmap for the first 256 characters combined with a vector of individual
wide characters, a vector of character ranges (for [A-Z] etc.), and a vector
of character classes (for [[:alpha:]] etc.).
One other point of interest is that although the Boyer-Moore algorithm had
to be disabled in the general multibyte case, it is still enabled for UTF-8
because of its self-synchronizing nature. This greatly speeds up matching
by reducing the number of multibyte conversions that need to be done.