add subbuf-rw, an lvalue subbuf like we have a substr-rw for strings

The replacement must be a Blob and this sub needs to be specced.
Add advent2011/day20 tests
Add skeleton CompUnit class
add unpack directives "a" and "Z"
Some more work on CompUnit, not yet functional
enable autounfudge of moar, ala JVM
run more spectest files

(all tested on moar)
wrap should not force decontainerization.
Make initial/max threads introspectable.

As requested by segomos++.
Ensure thread counts are Ints.
Remove hack for $NaN: constant: seems to work now
Get rid of the $Inf hack

Turns out +Inf breaks in the early settings somehow in the "mast" stage with:
Code ref does not exist in serialization context
Add CompUnit.name and fix .perl
Do searching for files in Perl 6

So do not use "locate_candidates" anymore.  This also allows smart matching
on module names to be selected.
Don't bind flattening things to a Scalar.

Relying on this dubious behavior prevents otherwise-successful case
analysis of Scalar assignment, forcing all assignments to scalars to
endure extra checks, bulking code-gen significantly on Moar.
Case-analyze $ sigil assignment.

We already did it for @ and %; this leaves sigilless as needing the
full-blown checks.
Use "$a is rw" to exploit better code-gen.

This replaces some "\a is rw" usages. When we know it's a scalar of
some kind, use $ to get simpler code-gen.
Get latest NQP with repeat until code-gen fix.

Also gets a MoarVM with some extra optimizations, including OSR.
fix 'fail' so it also prints a backtrace

This works in all cases except when the fail is the last statement of
our program. The call to .sink in main.nqp seems to introduce a problem,
which results in a low level backtrace that does not even show the file
and line number of the user's code.
fix Label.gist (thinko in how substr works)