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.
You should probably `@test` something too. This code will not verify anything other than that it doesn't throw exceptions. I know the results are random, but the size, type and range is not.