[packing] Packing Attempt #2

[btwj]

Mostly the same as the old implementation, with:

• No use of SsaBlockSplit for the case where a bool is viewed as an int. SsaBlockSplit breaks for constant conditions when used to rewrite SSA. I can't quite figure out how to modify SsaBlockSplit to handle the full generality of an arbitrary number of cases, so I just didn't use it in this case. All other instances of SsaBlockSplit are not constants, which is why this problem never arose. Regardless, we'll probably want a custom BoolViewI operator here.
• I added an optimization to remove some classes of type subsumption. Basically, it looks if a scalar is only used to represent a certain type and is never packed. If it is, then it uses that type in the representation instead of weaking to a u32[B32] or u64[B64] etc.
• Refactoring to package the solution up as a VariantSolution (which will have an associated score function to be implemented). For now, it still just outputs the first solution it finds.
• Prettier printing of variant norms and unboxing with the -print-packing flag:

[titzer]

LGTM