use vmap for all activations

i.e. extend #199 to all activations, now that Zygote implements the vmap adjoint.
Let's do proper benchmarking when doing this.

@AStupidBear

Start by reviewing issue #199 and the current activation implementations in NNlib.jl, then check how Zygote's vmap adjoint applies to each activation. Run the existing benchmarks before changing anything and compare results afterward; done means all activations use vmap with benchmark evidence recorded.

Written by the indexing model from the issue text.