Validate operator return types

[bjorn3]

Currently passing no return types for i32const and i32add passes validation, but results in a compilation panic.

[bjorn3]

Looks like argument count isn't validated either. At least for i32eq.

[bjorn3]

Nor is the alignment of memory operations being valid validated.

[bjorn3]

Another thing missing is checking for mismatches between block params and BlockTarget.

[cfallin]

These are all things that should be validated, indeed!

waffle was codeveloped with weval and so has not had a separate pass of robustification or API cleanup in general -- this is one example, where the validator was built to help debugging efforts but is not complete. I'm happy to review PRs that add any/all of the above!