Simd percent decoder (#458)

* incorporate simd percent decoder

* fixing some comments

* made avx512 version match existing behavior on malformed input

* made avx2 version match existing behavior on malformed input. made percent_decode_slow the fallback on non-x64_64 or x64_64 lacking avx2, which required changing the function signature of percent_decode_slow. deleted simde version since simde would require an external dependency.

* got rid of a few instructions in AVX2 version

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Co-authored-by: Jeff Plaisance <[email protected]>

Author: lemire