Add AVX2 `polyvec_{de,}compress` #2667

Job	Run time
Linting (ubuntu-latest)	1m 21s
Quickcheck (aarch64)	1m 21s
Compiler tests (aarch64)	1m 15s
Quickcheck (x86_64)	52s
Compiler tests (x86_64)	1m 16s
cbmc_k4	0s
Functional tests (${{ matrix.target.name }})	0s
cbmc_k3	0s
cbmc_k2	0s
Functional tests (${{ matrix.target.name }})	0s
	6m 5s

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