Implement the Wasm relaxed-simd proposal

[Robbepop]

This is about adding support for the relaxed-simd Wasm proposal.

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Now that the Wasm simd proposal is implemented for Wasmi, implementation of Wasm relaxed-simd is finally unblocked.

This is a rather small extension to the Wasm simd proposal that introduces some more efficient SIMD instructions that may have non-deterministic behavior on different platforms. However, since Wasmi is emulating SIMD instructions we should be able to make to implement all these SIMD instructions in a deterministic way.

The relaxed-simd proposal has been stable for a long time now, so it is about time it gets supported by Wasmi, especially with the new foundations.

In order to differentially fuzz test Wasmi's deterministic relaxed-simd implementation we can use Wasmtime's deterministic implementation using this config: Config::relaxed_simd_deterministic