Add `softmax` function #380

Job	Run time
tests/stable	2m 54s
tests/beta	3m 22s
tests/nightly	3m 47s
tests/1.64.0	4m 37s
clippy/stable	10s
cross_test/s390x-unknown-linux-gnu/stable	5m 56s
cargo-careful	2m 8s
cross_test/i686-unknown-linux-gnu/stable	1m 18s
format/nightly	11s
miri	8m 20s
nostd/thumbv6m-none-eabi/stable	17s
blas-msrv	43s
docs/nightly	19s
conclusion	1s
	34m 3s

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