Enable the multi-value proposal by default

[alexcrichton]

This was merged into the wasm spec upstream in WebAssembly/spec#1145, so
let's follow the spec and enable it by default here as well!

Some assorted updates happen here as well:

• The spec testsuite repo is updated to remove tests which assert that multi-value does not validate.
• The wasmparser dep is updated to bring in a few error message updates to pass the updated spec tests.

[github-actions]

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[fitzgen]

🎉