Add initial support for EVEX encodings

[abrown]

• [x] This has been discussed in issue #1244.
• [x] A short description of what this does, why it is needed: since the Wasm SIMD spec has re-added instructions (e.g. i64x2.mul) that are optimally encoded in VEX/EVEX, it seems like we should support these encoding formats. For this PR, only EVEX was added (and not every addressing form) in order to implement i64x2 multiplication. I haven't yet test-run this on an AVX512DQ-compatible CPU but will soon; while implementing this, I discovered that there is no good way to indicate an encoding that is predicated on either of two settings flags (see TODO in encodings.rs).
• [x] This PR contains test cases, if meaningful.
• [x] A reviewer from the core maintainer team has been assigned for this PR (I'm tagging people that might be interested in this).

[abrown]

@iximeow, thanks for all of the help! (And @peterhuene too!) I believe this is ready for an actual review--@iximeow, can you take a look? I rebased the commits into some semblance of order so they should be reviewable from oldest to newest (fun fact of the day: I had to redate them for GitHub to show them in order). Since we likely won't want to squash these things, I made sure the commits are bisectable by running git rebase master --exec 'cargo test --all' with no issues.

I would like to be able to actually run this new instruction on AVX512 hardware with a test run filetest but I will try that in a future PR; currently I don't think we have the ability to skip test run filetests when the target doesn't match the host's CPU features (or do we?).

[bnjbvr]

Closing in favor of https://github.com/bytecodealliance/wasmtime/pull/1192.