refactor: introduce helper function MAP_INPUT to instantiate MAP_INPUT_ functor

@amathad this is connected to the discussion yesterday about how to specify the template parameter.

I've talked to @nnolte and @rmatev about this a bit and then came up with this working draft.

That is, the original functor is renamed from MAP_INPUT to MAP_INPUT_ and then we create a helper function which we name MAP_INPUT.
The user can now just use this function as if it is a normal functor.

I had to make some minimal tweaks to the automatic doc generation which I can make an MR in Moore for but after that the doc looks like:

Which I think looks good. (aside from the fact that template parameters aren't being picked up, but that's a different problem)

Let me know you think.