[packing] [WIP] Simple packing

[btwj]

Progress on packing fields in a variant.

• Introduces IntRepType that extends IntType. Allows for values of this type to undergo bitwise operations with the ordinary operators.

For now, I've introduced a #packed annotation to tell the compiler to pack a variant, while I'm still trying to figure out the best semantics for the #packing annotation.

TODOs

• [x] Share code between tagless and tagged variants so that packing works for both
• [ ] Add escape hatch for when solver is unable to find a solution - error out and report to user

[titzer]

Can you bump the version number and I'll land this?