wasm32: Add relaxed simd instructions

This commit adds intrinsics to the `wasm32` to support the [relaxed SIMD
proposal][proposal]. These are added with the same naming conventions of
existing simd-related intrinsics for wasm which is similar to the
instruction name but matches sign in a few places.

This additionally updates Wasmtime to execute tests with support for the
relaxed simd proposal. No release has been made yet so this uses the
`dev` release, and I can make a PR in April when the support in Wasmtime
has been released to an official release. The `wasmprinter` crate is
also updated to understand these instruction opcodes as well.

Documentation has been added for all intrinsics, but tests have only
been added for some of them so far. I hope to follow-up later with more
tests.

[proposal]: https://github.com/WebAssembly/relaxed-simd

Author: alexcrichton

Diff for: ci/docker/wasm32-wasi/Dockerfile

@@ -7,11 +7,10 @@ RUN apt-get update -y && apt-get install -y --no-install-recommends \
   xz-utils \
   clang
 
-RUN curl -L https://github.com/bytecodealliance/wasmtime/releases/download/v0.29.0/wasmtime-v0.29.0-x86_64-linux.tar.xz | tar xJf -
-ENV PATH=$PATH:/wasmtime-v0.29.0-x86_64-linux
+RUN curl -L https://github.com/bytecodealliance/wasmtime/releases/download/dev/wasmtime-dev-$(arch)-linux.tar.xz | tar xJf -
+ENV PATH=$PATH:/wasmtime-dev-$(arch)-linux
 
 ENV CARGO_TARGET_WASM32_WASI_RUNNER="wasmtime \
-  --enable-simd \
-  --enable-threads \
+  --wasm-features=threads,relaxed-simd \
   --mapdir .::/checkout/target/wasm32-wasi/release/deps \
   --"

Diff for: crates/core_arch/src/wasm32/mod.rs

@@ -9,6 +9,9 @@ pub use self::atomic::*;
 mod simd128;
 pub use self::simd128::*;
 
+mod relaxed_simd;
+pub use self::relaxed_simd::*;
+
 mod memory;
 pub use self::memory::*;