faster randmtzig_exprnd (separate unlikely branch + inline) and rename it "randexp"

Same transformation as was done recently on `randn`.
The rename was suggested by @ViralBShah. It turns out that this function is not exported by Base (nor documented it seems), should it be? If yes, I will write the array version `randexp!` too.
Here I only duplicated what already existed for `randn`, but I agree that testing values should be added.
I personally find that it is good to test somewhere only the API (smaller sizes than 100000 would do it though). What we could do here is test that the global version and the version with a passed rng give the same results. I am planning to rewrite #8339 differently, with all API checks grouped together. Then it could be enough to do all the other tests on the global RNG.