Start translating multi-dispatch to NQP.

Add an NQP version of the candidate narrowness comparator function.
Port multi candidate sort algorithm to NQP.

Not used yet.
Start porting multi-dispatcher to NQP.

Some of the complex bits are commented out, and it doesn't quite work
yet, but this gets the overall algorithm translated.
Various fixes.

Gets the NQP-based multi-dispatcher working for some basic cases.
Merge branch 'nom' into md-nqp
Named args and constraints in NQP-based disptacher
Give NQP-based dispatcher support for 'is default'
Get junction failover into multi-dispatcher.
Correct sentinel.
Invalidate sort order as needed.
Fix compiler thunk checking.
Switch most dispatches to NQP-based dispatcher.

Next up will be hunting/fixing regressions.
Allow binding to $/.

Because we need it now to fix other things, but also because it will
fix an RT once this branch is merged.
Fix $/ setting and $_ lookup a bit.

We "got lucky" because the callframe of the proto was magically going
missing, thanks to an optimization. This should resolve it, for now at
least, though it feels like we're missing a more real solution.
Fix $/ lookup more properly.
A couple of performance tweaks.
Add back caching of dispatches.
Need to auto-thread shouldn't count as failure.

Fixes auto-threading regression with NQP-based multi-dispatch.
Remove now-unused mappings.
Don't build error strings just to discard them.