Proposal: add `on` to the Prelude

[Henning Thielemann]

On Tue, 10 Sep 2019, David Feuer wrote:

> Indeed, there are a lot more conflicts than I'd have expected. Ignoring 
> functions with the same type, Hoogle shows this name in the below 
> packages.

It's not only about top-level functions named 'on'. If I use 'on' as a 
local variable, then I get warnings about local redefinitions of the 
global Prelude.on.