Enabled merged proposals by default (#1405)
This enables three proposals by default since they've been merged into
the upstream specification:
* `saturating-float-to-int` - WebAssembly/spec#1143
* `sign-extension` - WebAssembly/spec#1144
* `multi-value` - WebAssembly/spec#1145
Most of the fallout from this is in the test suite with lots of
`--enable` flags getting removed and some tests which now
unconditionally pass also getting removed. Two spec tests explicitly
pass `--disable` until the spec test submodule is updated.
