[MERGE] revamp dist/B-Deparse/t/core.t

rewrite core.t to be a comprehensive test of most keywords
along with various lengths of args of the form foo($a, $b, $c),
where $a etc may or may not be lexical.

(Previously this test file only checked that CORE::foo deparsed as
CORE::foo when sub foo was defined: it generated args, but didn't check
how they were deparsed).

This branch also includes a number of fixups to Deparse.pm that were found
while working on core.t. Note there are a few places in core.t marked
with 'XXX' where there is some dodgy stuff going on that core.t just
ignores for now.