iter: add GOEXPERIMENT=rangefunc-limited version [freeze exception]

[rsc]

CL 543319 needs to be submitted for the release.

[heschi]

cc @golang/release

[mknyszek]

@cherrymui and @heschi both looked at https://go.dev/cl/543319 and concluded that it has no effect with the experiment disabled, so it's safe to land after the freeze.

@rsc in the release meeting, we'd appreciate it if you could elaborate on the justification in the issue, but otherwise the freeze exception is tentatively accepted.

[rsc]

The justification is that this change is necessary for people to try GOEXPERIMENT=rangefunc and see the high-performance iter.Pull that we expect rather than the low-performance one that can be implemented without runtime changes. The changes are limited to the implementation of iter.Pull - if you're not using it, there is no risk. And the change are only visible when using GOEXPERIMENT=rangefunc - if you don't set it, you can't get at any of this code. So the risk to the overall release is as close to zero as a change can be.

[cagedmantis]

As per discussion in the release meeting, this has been approved.

[cherrymui]

CL submitted.