Rollup merge of #139533 - jogru0:130711, r=Mark-Simulacrum

add next_index to Enumerate

Proposal: rust-lang/libs-team#435
Tracking Issue: #130711

This basically just reopens #130682 but squashed and with the new function and the feature gate renamed to `next_index.`

There are two questions I have already:
- Shouldn't we add test coverage for that? I'm happy to provide some, but I might need a pointer to where these test would be.
  - Maybe I could actually also add a doctest?
- For now, I just renamed the feature name in the unstable attribute to `next_index`, as well, so it matches the new name of the function. Is that okay? And can I just do that and use any string, or is there a sealed list of features defined somewhere where I also need to change the name?

Author: ChrisDenton