* Enable the SIMD proposal by default
This commit updates Wasmtime to enable the SIMD proposal for WebAssembly
by default. Support has been implemented for quite some time and
recently fuzzing has been running for multiple weeks without incident,
so it seems like it might be time to go ahead and enable this!
* Refactor CLI feature specification
Don't store a `bool` but rather an `Option<bool>` so we can inherit the
defaults from Wasmtime rather than having to keep the defaults in-sync.