Add AVX2 `polyvec_{de,}compress` #2668

Job	Run time
Linting (ubuntu-latest)	1m 22s
Quickcheck (aarch64)	54s
Quickcheck (x86_64)	1m 1s
Compiler tests (aarch64)	1m 17s
Compiler tests (x86_64)	1m 17s
Functional tests (${{ matrix.target.name }})	0s
Functional tests (${{ matrix.target.name }})	0s
cbmc_k2	0s
cbmc_k3	0s
cbmc_k4	0s
	5m 51s

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