Scalar container lands.

Scalar container support has landed to PIL^N runtime.  To wit:

$ make pil
$ ./pil -e '::Scalar`create(3).STORE(9).FETCH()'
### Evaluated ###
9

Moreover, ::Int now represents a true BigInt (internally using the Haskell type Integer), which differs from its unboxed representation with limited precision.  This is correct according to the spec, and unlike the old runtime which uses BigInts everywhere.  Unicode-aware ::Str types will get the same treatment soon.

The next step is to implement Array and Hash container types, and revise PIL² so they can be desugared to PIL^N. Which should all happen tomorrow, unless I get distracted into more Rules hacking...