feat(simd): I8/I16 SIMD vectors + slice-level int ops (#124, sprint A3) #225

Job	Run time
clippy/stable	38s
nostd/thumbv6m-none-eabi/stable	36s
native-backend/stable	54s
Pass MSRV values to other jobs	4s
miri	30s
cross_test/s390x-unknown-linux-gnu/stable	2m 4s
format/stable	32s
docs/nightly	54s
cross_test/i686-unknown-linux-gnu/stable	1m 44s
cargo-careful	35s
blas-msrv	1m 5s
tests/beta	1m 13s
tests/1.64.0	52s
tests/stable	1m 9s
conclusion	0s
	12m 50s

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