enum TxfmSize: Make a real enum

[kkysen]

• Part of fn decode_coefs is slow #1180.

This makes TxfmSize and RectTxfmSize into a real enum, which eliminates many bounds checks in hot fns like fn decode_ceofs. It also lets us further simplify the itx macros and eliminate the distinction between the Rect ones.

Note that TxfmSize defined the square sizes; these ones are named with an S prefix, while the R-prefixed RectTxfmSize stay R-prefixed.

There are a few new places where we have to create TxfmSizes using from_repr and thus do a bounds check. These should be pretty cheap, but there are optimizable out, as they ultimately come from static/const data whos bounds are known but LLVM is not able to deduce them. This is why I added 7f0cdc2, starting to try to fix this, though it's a little complicated, so I didn't want to finish it in this PR.