BTreeMap: move generic support functions out of navigate.rs

[ssomers]

A preparatory step chipped off #78104, useful in general (if at all).

r? @Mark-Simulacrum

[ssomers]

Note this would be another benign conflict postponing #78015 (because of the alphabetically close new module names), but to me it seems sensible to do it first.

[bors]

☔ The latest upstream changes (presumably #78015) made this pull request unmergeable. Please resolve the merge conflicts.

Note that reviewers usually do not review pull requests until merge conflicts are resolved! Once you resolve the conflicts, you should change the labels applied by bors to indicate that your PR is ready for review. Post this as a comment to change the labels:

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

[Mark-Simulacrum]

Oh, hm, I did not see this before that other PR. If you want to ping me on Zulip to just say "please review in this order" or whatever, that's fine by me -- otherwise I usually go in oldest first order in my waiting-on-review queue.

[Mark-Simulacrum]

r=me though on the change itself (I think I approved a similar commit elsewhere, IIRC a bit smaller but I might be misremembering)

[ssomers]

@rustbot modify labels: +S-waiting-on-review -S-waiting-on-author

Still feeding this commit separately, because it's simple and logically distinct, even though you approved the rest of #78104 (without the no-rollup constraint, to my surprise, but I'm just beginning to understand that process).

[Mark-Simulacrum]

@bors r+ rollup=never

[bors]

📌 Commit 0da7941 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 0da7941 with merge c9b606e...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing c9b606e to master...