Convert B to use ExtUtils::Constant::ProxySubs for its constants.

Convert B to use ExtUtils::Constant::ProxySubs for its constants.

Previously it was using a Perl script to generate C code with pairs of
 newCONSTSUB(stash,"Foo_BAR",newSViv(Foo_BAR));
 av_push(export_ok,newSVpvn("Foo_BAR",7));
for each constant it exported from C to Perl.

Now it uses a loop to process a table. Object code is size reduced by about 42K
on this system (almost 20%)