make <= 1-word types immediate

[thestinger]

I fixed newtype immediates in #9643 while working on making tuples immediate but there's more to fix than just flipping it on for other types.

[thestinger]

#9656 does this for tuples

[thestinger]

#9677 does this for enums, except for C-like ones

[thestinger]

#9699 does unit structs and C-like enums

[thestinger]

Small ty_struct is the only thing left to do, at least before increasing the threshold to a value queried from LLVM (slices, traits, closures, etc. could be immediate on some targets).

It hits a raw uncaught exception at the moment.