Admit AVX2 encode backend #254

Job	Run time
Miri	29m 29s
Platform tests (windows-latest)	39s
no_std target build (aarch64-unknown-linux-gnu)	22s
Platform tests (macos-15)	41s
Platform tests (macos-latest)	29s
no_std target build (x86_64-unknown-freebsd)	22s
Platform tests (macos-15-intel)	2m 14s
Format, lint, test, and audit	1m 30s
no_std target build (x86_64-unknown-linux-gnu)	16s
Platform tests (ubuntu-latest)	32s
Platform tests (macos-26)	44s
no_std target build (thumbv7em-none-eabihf)	18s
no_std target build (wasm32-unknown-unknown)	23s
	37m 59s

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