Implements `table.get`, `table.set`, `elem.drop`, `table.size`,
and `table.grow`. Also fixes a few issues when generating ref-related
spectests. Also changes the `TableInstance` type to use
`Vector<Reference>` instead of `Vector<Optional<Reference>>`, because
the ability to be null is already encoded in the `Reference` type.
...and let LibWasm do the validation instead of removing the test when a
module is invalid.
Also, one of the tests has an integer literal starting with zero, so
account for this to make it not fail :^)
`wasm-as` will do some semantic analysis on the modules, which is not
something we're looking for here.
Instead, use `wat2wasm` to generate the exact module.
This also optionally generates a test suite from the WebAssembly
testsuite, which can be enabled via passing `INCLUDE_WASM_SPEC_TESTS`
to cmake, which will generate test-wasm-compatible tests and the
required fixtures.
The generated directories are excluded from git since there's no point
in committing them.
