Auto merge of #100845 - timvermeulen:iter_compare, r=scottmcm

Use internal iteration in `Iterator` comparison methods

Updates the `Iterator` methods `cmp_by`, `partial_cmp_by`, and `eq_by` to use internal iteration on `self`. I've also extracted their shared logic into a private helper function `iter_compare`, which will either short-circuit once the comparison result is known or return the comparison of the lengths of the iterators.

This change also indirectly benefits calls to `cmp`, `partial_cmp`, `eq`, `lt`, `le`, `gt`, and `ge`.

Unsurprising benchmark results: iterators that benefit from internal iteration (like `Chain`) see a speedup, while other iterators are unaffected.
```
 name                           before ns/iter  after ns/iter  diff ns/iter   diff %  speedup
 iter::bench_chain_partial_cmp  208,301         54,978             -153,323  -73.61%   x 3.79
 iter::bench_partial_cmp        55,527          55,702                  175    0.32%   x 1.00
 iter::bench_lt                 55,502          55,322                 -180   -0.32%   x 1.00
```

Author: bors