Better discriminant types, transmute special cases, onInt attributes

[jberthold]

• The rvalueDiscriminant operation was unconditionally returning 128-bit integers, which turns out to be wrong. We now consult the layout to return the correct bit width (but still always unsigned as this seems to be an invariant).
  Closes Inconsistent uint types in proofs initialize_account* solana-token#41
• Two special cases of transmute are implemented, removing thunks that transmute would have created before.
  • round-trip transmutes through a given type back to the original type are eliminated
  • for a transmute from an Integer to an enum without fields followed by a discriminant operation, the original integer value is returned as the discriminant.
• the (partial) onInt function was missing preserves-definedness attributes on its equation so booster did not apply it

[dkcumming]

I think this is good, no changes necessary from what I can see.