Rely on wasmparser for validation

[alexcrichton]

This commit removes the type-level validation performed within each
function that walrus previously did. Long-ago in the AST-based IR this
sort of made more sense but nowadays walrus does very little other than
simply validate the input. Recent refactorings in wasmparser are
targeted at reuse of the internal validator, so this commit does exactly
that, reuses wasmparser's validator and removes the type-level
validation in walrus.

[fitzgen]

Nice! Twice as many deletions as additions :)