feat(simd): Tier 3 U16x32 + movemask + Dockerfile/CI AVX2 default + docs #193

Job	Run time
native-backend/stable	45s
Pass MSRV values to other jobs	4s
clippy/stable	28s
format/stable	26s
nostd/thumbv6m-none-eabi/stable	35s
cross_test/s390x-unknown-linux-gnu/stable	1m 47s
cross_test/i686-unknown-linux-gnu/stable	1m 50s
cargo-careful	0s
miri	0s
docs/nightly	0s
blas-msrv	1m 32s
tests/1.64.0	56s
tests/beta	1m 1s
tests/stable	1m 6s
conclusion	0s
	10m 30s

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