[stdlib] Add parameter for generator for UInt

[martinvuyk]

Part of #4283

[JoeLoser]

Can you add some simple unit tests to show it works as expected for the various cases, please?

[martinvuyk]

@JoeLoser You were right wanting to add tests. It turns out the loop is returning Int instead of UInt. I imagine that at some point the UInt yield type gets lowered to __mlir_type.index/changed to IntLiteral and the default path for reconstruction is then through Int. I think this would dovetail nicely with an idea I've been having: I know it might get nasty, but maybe we can create "conditional materialization" where IntLiteral lowers to an Int when it is negative and UInt otherwise.