perf: fix SIMD-inlining

[AaronO]

Drastically improving throughput on larger inputs (3x+ for large URIs or header-values)

There are 2 optimizations in this PR:

1. Removing two unnecessary instructions when computing trailizing_zeros / bytes-validated. We don't need to or the upper half of the register with xFF we can instead compute trailing-zeros on the meaningful bits by using eax (u32) instead of rax (u64) and ax (u16) instead of eax (u32) for AVX2 and SSE4.2 respectively.
2. Correctly scoping target_feature pragmas to allow SIMD validators to be inlined, so when looped we benefit from greater register reuse etc... See:
   • https://doc.rust-lang.org/reference/attributes/codegen.html#the-target_feature-attribute
   • https://rust-lang.github.io/rfcs/2045-target-feature.html

Benchmarks

Summary table

(Disclaimer: aggregated by ChatGPT, which "computed" the ratio rows which aren't exactly correct but close enough)

Test	128b	256b	512b	1024b	2048b	4096b
Before
Header	38	66	123	263	484	946
URI	19	44	116	237	465	937
After
Header	30	39	55	88	193	300
URI	12	20	35	65	127	270
Improvement
Header Ratio	~1.5x	~1.5x	~2.0x	~3.0x	~2.5x	~3.0x
URI Ratio	~1.5x	~2.0x	~3.5x	~3.5x	~3.5x	~3.5x

Raw benches

before:
test header/value_128b ... bench:           38 ns/iter (+/- 3)
test header/value_256b ... bench:           66 ns/iter (+/- 0)
test header/value_512b ... bench:           123 ns/iter (+/- 2)
test header/value_1024b ... bench:          263 ns/iter (+/- 13)
test header/value_2048b ... bench:          484 ns/iter (+/- 19)
test header/value_4096b ... bench:          946 ns/iter (+/- 7)

test uri/uri_128b ... bench:          19 ns/iter (+/- 3)
test uri/uri_256b ... bench:          44 ns/iter (+/- 1)
test uri/uri_512b ... bench:         116 ns/iter (+/- 1)
test uri/uri_1024b ... bench:         237 ns/iter (+/- 3)
test uri/uri_2048b ... bench:         465 ns/iter (+/- 3)
test uri/uri_4096b ... bench:         937 ns/iter (+/- 58)

after:
test header/value_128b ... bench:           30 ns/iter (+/- 1)
test header/value_256b ... bench:           39 ns/iter (+/- 1)
test header/value_512b ... bench:           55 ns/iter (+/- 2)
test header/value_1024b ... bench:          88 ns/iter (+/- 4)
test header/value_2048b ... bench:          193 ns/iter (+/- 49)
test header/value_4096b ... bench:          300 ns/iter (+/- 4)

test uri/uri_128b ... bench:          12 ns/iter (+/- 3)
test uri/uri_256b ... bench:          20 ns/iter (+/- 0)
test uri/uri_512b ... bench:          35 ns/iter (+/- 1)
test uri/uri_1024b ... bench:          65 ns/iter (+/- 4)
test uri/uri_2048b ... bench:         127 ns/iter (+/- 2)
test uri/uri_4096b ... bench:         270 ns/iter (+/- 36)