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.
Instead of using the `runtime` bool, you could use `static if (is(Fun[0]))` to distinguish a function type from an aliased function.