Teach Perl about Unicode named character sequences

Teach Perl about Unicode named character sequences

mktables is changed to process the Unicode named sequence file.
charnames.pm is changed to cache the looked-up values in utf8.  A new
function, string_vianame is created that can handle named sequences, as
the interface for vianame cannot.  The subroutine lookup_name() is
slightly refactored to do almost all of the common work for \N{} and the
vianame routines.  It now understands named sequences as created my
mktables..

tests and documentation are added.  In the randomized testing section,
half use vianame() and half string_vianame().