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
Added .distance to Str and made Smart Match against tr/// it appropriately.