* Update representation of `v128`
* Rename everything with new naming convention of underscores and no
modules/impls
* Remove no longer necessary `wasm_simd128` feature
* Remove `#[target_feature]` attributes (use `#[cfg]` instead)
* Update `assert_instr` tests
* Update some implementations as LLVM has evolved
* Allow some more esoteric syntax in `#[assert_instr]`
* Adjust the safety of APIs where appropriate
* Remove macros in favor of hand-coded implementations
* Comment out the tests for now as there's no known runtime for these
yet
Turns out Cargo doesn't automatically set `TARGET` for rustc invocations so
carry it forward manually from the build script over to the rustc invocation.
