Remove loop's result value(s).

[sunfishcode]

Following up on discussion in #741, this PR proposes removing loop's exit value. If branch arities are moved to block and if as proposed, this change eliminates the need for loop to also have an arity immediate, which would simplify a common validation strategy.

I expect this change to have little negative effect on code size or expressiveness, because (loop x y) could usually be written as (loop x) y, and the change would also eliminate an immediate field on every loop.

[titzer]

This mostly looks good to me, but I think it does depend on the outcome of #741. It's probably the case the value-returning loops is quite rare, and in that case one can surround the loop with a block that does return a value and branch to it.

[lukewagner]

+1

[sunfishcode]

The current evolution of block signatures has them applying to loop as well, making this change less important.