Add Iterator comparison methods that take a comparison function

This PR adds `Iterator::{cmp_by, partial_cmp_by, eq_by, ne_by, lt_by, le_by, gt_by, ge_by}`. We already have `Iterator::{cmp, partial_cmp, ...}` which are less general (but not any simpler) than the ones I'm proposing here.

I'm submitting this PR now because rust-lang#61505 has been merged, so this change should not have a noticeable effect on the `Iterator` docs page size.

The diff is quite messy, here's what I changed:
- The logic of `cmp` / `partial_cmp` / `eq` is moved to `cmp_by` / `partial_cmp_by` / `eq_by` respectively, changing `x.cmp(&y)` to `cmp(&x, &y)` in the `cmp` method where `cmp` is the given comparison function (and similar for `partial_cmp_by` and `eq_by`).
- `ne_by` / `lt_by` / `le_by` / `gt_by` / `ge_by` are each implemented in terms of one of the three methods above.
- The existing comparison methods are each forwarded to their `_by` counterpart, passing one of `Ord::cmp` / `PartialOrd::partial_cmp` / `PartialEq::eq` as the comparison function.

The corresponding `_by_key` methods aren't included because they're not as fundamental as the `_by` methods and can easily be implemented in terms of them. Is that reasonable, or would adding the `_by_key` methods be desirable for the sake of completeness?

I didn't add any tests – I couldn't think of any that weren't already covered by our existing tests. Let me know if there's a particular test that would be useful to add.

…ichton

Use backtrace formatting from the backtrace crate

r? @alexcrichton

resolve: Mark more erroneous imports as used

Fixes rust-lang#63724
r? @estebank

… r=GuillaumeGomez

rustdoc: fix diagnostic with mixed code block styles

This fixes a relatively obscure issue where the diagnostic (emitted [here](https://github.com/rust-lang/rust/blame/ef54f57c5b9d894a38179d09b00610c1b337b086/src/librustdoc/passes/check_code_block_syntax.rs#L69)) would get confused since the "is_fenced" flag wasn't reset properly.