Expose some MOP primitives for Perl 6 programmers.

Now the 6model API has been stable for a while, we should expose some
key aspects of it. So far, short of using nqp::ops, it has not been
possible for a Perl 6 meta-programmer to totally create their own
kinds of type from scratch; customizing existing ones has been the
only way. Custom publishing of method caches has also not been on
offer, and some modules could certainly make use of it. Now it will
be possible.
Fix missing decont with .REPR implementation.
Run S12-meta/primitives.t.
Make foo[] and foo{} consistent
allow $x ~~ s/// to return Match or list of Match

s/// is now implemented in terms of .subst-mutate, which returns
the matches instead of the result, just as normal matching does.
Back out adding Metamodel::Primitives on Parrot.

It breaks the build, for a likely hard-to-debug reason.
default to uname if it exists

The previous code assumed all Unix distributions could eventually be
enumerated somehow, which is delusional.  :)
Remove non-builtins from deftrap

It clutters up deftrap, and a new Perl 6 user might attempt something
like 'stat', and when they get the error message and they try '.stat'
and don't get the result they expect, it could be very confusing.
Bump NQP_REVISION for latest MoarVM.

Includes fixes for a qqx related crash that often showed up on OSX.
things that go bump in the nqp
Implement FALLBACK.
Run S12-methods/fallback.t.
Tentatively exclude somes names from FALLBACK.

This feels worryingly like a "list to remember". On the other hand, it
could become a list copy-pasted into everyone's code in order to make
FALLBACK to what people will probably want. Go this way for now; open
to something better in the future.
Tentatively unbreak parrot / jvm builds
Changelog: mention a few changes
mention DIVIDE_NUMBERS optimizations in changelog
update my CREDITS entry
ChangeLog tweaks
This month's release will be called 'Cologne'

In appreciation of this week's very successful Niederrhein.PM meeting.
Mention fixing of OS X flakiness