Add `softmax` function #377

Job	Run time
nostd/thumbv6m-none-eabi/stable	12s
clippy/stable	35s
blas-msrv	47s
cargo-careful	2m 3s
tests/stable	30s
cross_test/s390x-unknown-linux-gnu/stable	6m 33s
tests/beta	30s
cross_test/i686-unknown-linux-gnu/stable	1m 30s
docs/nightly	18s
tests/nightly	33s
format/nightly	10s
tests/1.64.0	46s
miri	8m 11s
conclusion	1s
	22m 39s

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