Feature/const generics

[calebzulawski]

This PR adds a few things:

• Switch to generic-width vectors!
• Overhaul masks (adds an opaque mask type with arbitrary layout, and a trait for using particular layouts)
• Adds comparison operations (supersedes PR Feature/comparisons #44)

Quite a bit is going on in this PR, so it might be worth building the docs and reading those first to get a hold of the interface, then moving on to the implementation.

[programmerjake]

Looks good to me!

[KodrAus]

Looks like we're hitting a bug in the codegen:

Invalid bitcast

  %4 = bitcast <4 x i32> %1 to [4 x i32], !dbg !272

in function _ZN9core_simd11vectors_i3216SimdI32$LT$_$GT$8to_array17h1010e6a6315f7e36E

LLVM ERROR: Broken function found, compilation aborted!

[calebzulawski]

I think this is the same issue from #48. I'm pretty sure it's a compiler bug and not our fault. Is the bitcast invalid because the source and destination types are the same?

[bjorn3]

The source is a vector I think, the destination is an array.

[calebzulawski]

It looks like arrays don't cast? This doesn't compile at all (https://llvm.godbolt.org/z/4xM1MY):

define [4 x i32] @cast([4 x float] %x) local_unnamed_addr #0 {
    %out = bitcast [4 x float] %x to [4 x i32]
    ret [4 x i32] %out
}

The equivalent with vectors works just fine.

[calebzulawski]

Opened rust-lang/rust#80108 to address the error. Going to try to find a workaround :)