Add `softmax` function #379

Job	Run time
format/nightly	11s
cargo-careful	2m 3s
blas-msrv	37s
clippy/stable	13s
cross_test/s390x-unknown-linux-gnu/stable	7m 35s
docs/nightly	20s
cross_test/i686-unknown-linux-gnu/stable	1m 25s
miri	8m 15s
tests/stable	1m 56s
tests/beta	1m 50s
tests/nightly	1m 40s
tests/1.64.0	1m 55s
nostd/thumbv6m-none-eabi/stable	8s
conclusion	1s
	28m 9s

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