perf: AVX2 8888 load/store + f32x16 save specializations

[bhark]

This PR is part of a series linked to #174.

What

AVX2 specializations for these primitives:

• lowp::load_8888 / lowp::store_8888 (these are the bulk of source_over_rbga)
• lowp::load_8
• highp::load_8888 / highp::store_8888
• f32x16::save_to_u16x16

Gated on feature = "simd", target_feature = "avx2". So SSe2/SSE4.1/AVX, Neon WASM-SIMD etc. are unchanged.

Why

These primitives, on AVX2, can collapse a long scalar shuffle into a handful of instrinsics. This means lowp::source_over_rgba drops from ~350 to about 50 instructions.

Results

	speedup
Geomean (55 benches)	1.95x
blend::destination_atop	3.83x
gradients::two_stops_linear_pad	3.56x
blend::source_over	2.53x
fill::rect	2.04x
gradients::three_stops_linear_even	1.92x
hairline::aa	1.34x
blend::clear, fill::opaque (don't hit specialized paths)	1.00x

I found no regressions anywhere.

Notes

Some nuances to this:

• lowp::store_8888 AVX2 ORs channels instead of truncating with as u8. Shouldn't be an issue, unless something produces channel values > 255 in lowp, in which case AVX2 would surface a visual artifact instead of truncating silently.
• Inspired style-wise by the existing src/wide/i32x8_t.rs / u32x8_t.rs patterns
• Default cargo builds naturally won't see this. You'll have to build with -Ctarget-cpu=haswell, -Ctarget-feature=+avx2 or target-cpu=native.

[RazrFalcon]

Looks good, but I would avoid direct SIMD intrinsics calls and cfg_if in highp/lowp code. All of them should be nicely wrapped in the wide module.

[bhark]

@RazrFalcon And there's that too, is this more aligned with what you had in mind?

[RazrFalcon]

Let's use global use instead. Like we do in u32x4_t.rs

[bhark]

Moved to module scope now. Also applied to u16x16_t.rs to keep things consistent.

[RazrFalcon]

Yes, looks much better now. The idea is too hide all low-level simd stuff in the wide module.

[RazrFalcon]

Good. Hopefully someone from the linebender team would merge it soon. @nicoburns ?
I think we can merge #172, #175, #176 and #177