Use neg intrinsics

[workingjubilee]

simd_neg was added in rust-lang/rust#83271 so now we get to use it.
This PR fixes #32.

[calebzulawski]

It looks like this is failing on wasm+simd?

Error: failed to deserialize wasm module

Caused by:
    0: failed to parse code section
    1: Unknown 0xfd opcode (at offset 576984)
error: test failed, to rerun pass '--test i16_ops'
Error: Running Wasm tests with wasm-bindgen-test failed
Caused by: failed to execute `cargo test`: exited with exit code: 1
  full command: "cargo" "test" "--release" "--target" "wasm32-unknown-unknown"
The command "wasm-pack test --chrome --headless crates/core_simd --release" exited with 1.

0xfd are the SIMD opcodes. Not sure if this is a bug in LLVM or chrome

[workingjubilee]

One way to help tell would be to add the ability to test against wasmtime and/or Firefox SIMD for reference.

[calebzulawski]

I wonder if something broke in chrome, since #83 is failing for the same reason.