Introduce std.range.generate

`generate` (name up to debate) simply takes a callable, and turns it into a range. Particularly useful for things like random number generators, functions with state, or functions that modify a global state.

As requested in the forums. Pinging @MartinNowak.
Given __a__ callable ($(XREF __traits__, isCallable)) $(D fun) create __an infinite__ range whose front method forwards to $(D fun).