Add `softmax` function #376

Job	Run time
nostd/thumbv6m-none-eabi/stable	9s
format/nightly	13s
clippy/stable	10s
tests/stable	25s
blas-msrv	24s
tests/beta	21s
cargo-careful	2m 7s
tests/nightly	28s
cross_test/s390x-unknown-linux-gnu/stable	1m 39s
tests/1.64.0	36s
cross_test/i686-unknown-linux-gnu/stable	1m 25s
docs/nightly	21s
miri	8m 19s
conclusion	1s
	16m 38s

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