Add `softmax` function #381

Job	Run time
cargo-careful	2m 9s
clippy/stable	12s
cross_test/s390x-unknown-linux-gnu/stable	6m 15s
blas-msrv	44s
cross_test/i686-unknown-linux-gnu/stable	1m 24s
docs/nightly	21s
format/nightly	13s
miri	8m 38s
nostd/thumbv6m-none-eabi/stable	30s
tests/stable	2m 48s
tests/beta	2m 48s
tests/nightly	3m 45s
tests/1.64.0	4m 47s
conclusion	1s
	34m 35s

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