BTree: move more shared iterator code into navigate.rs

[ssomers]

The functions in navigate.rs only exist to support iterators, and these look easier on my eyes if there is a shared struct with the recurring pair of handles.

r? @Mark-Simulacrum

[ssomers]

benchcmp old new --threshold 10
 name                                           old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 btree::map::first_and_last_0                   10           12                      2   20.00%   x 0.83
 btree::map::first_and_last_10k                 60           68                      8   13.33%   x 0.88
 btree::map::iter_1                             11,139       7,022              -4,117  -36.96%   x 1.59
 btree::map::iter_100                           12,293       7,775              -4,518  -36.75%   x 1.58
 btree::map::iter_10k                           13,294       9,029              -4,265  -32.08%   x 1.47
 btree::map::iter_1m                            14,408       9,924              -4,484  -31.12%   x 1.45
 btree::map::range_included_included            418,226      503,045            84,819   20.28%   x 0.83
 btree::map::range_unbounded_vs_iter            126,138      81,175            -44,963  -35.65%   x 1.55
 btree::set::difference_random_100_vs_10k       3,179        2,443                -736  -23.15%   x 1.30
 btree::set::difference_staggered_100_vs_10k    2,236        2,743                 507   22.67%   x 0.82
 btree::set::intersection_random_100_vs_10k     2,779        2,271                -508  -18.28%   x 1.22
 btree::set::intersection_random_10k_vs_100     2,785        2,251                -534  -19.17%   x 1.24
 btree::set::intersection_staggered_100_vs_10k  2,039        2,550                 511   25.06%   x 0.80
 btree::set::is_subset_100_vs_10k               1,180        1,639                 459   38.90%   x 0.72

[bors]

☔ The latest upstream changes (presumably #81486) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #81956) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #82103) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

@bors r+ rollup=never p=0

[bors]

📌 Commit 342aa69 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 342aa69 with merge 3cc54d0993aa1b56887dce7fcb95ed279a79dba2...

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

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Build completed successfully in 0:01:03
+ /scripts/validate-toolstate.sh
Cloning into 'rust-toolstate'...
<Nothing changed>
error: miri maintainer @oli-obk is not assignable in the rust-lang/rust repo
error: rustfmt maintainer @topecongiro is not assignable in the rust-lang/rust repo
error: nomicon maintainer @Gankra is not assignable in the rust-lang/rust repo
error: reference maintainer @matthewjasper is not assignable in the rust-lang/rust repo
error: reference maintainer @Havvy is not assignable in the rust-lang/rust repo
error: reference maintainer @ehuss is not assignable in the rust-lang/rust repo
error: embedded-book maintainer @therealprof is not assignable in the rust-lang/rust repo
error: edition-guide maintainer @ehuss is not assignable in the rust-lang/rust repo
error: rustc-dev-guide maintainer @amanjeev is not assignable in the rust-lang/rust repo

  To be assignable, a person needs to be explicitly listed as a
  collaborator in the repository settings. The simple way to
  fix this is to ask someone with 'admin' privileges on the repo
  to add the person or whole team as a collaborator with 'read'
  privileges. Those privileges don't grant any extra permissions
  so it's safe to apply them.
The build will fail due to this.

[bors]

💔 Test failed - checks-actions

[ehuss]

@bors retry
error: miri maintainer @oli-obk is not assignable in the rust-lang/rust repo

[bors]

⌛ Testing commit 342aa69 with merge b10e76598262dd4ef9112cdc9a00e79126e37f0d...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[ehuss]

@bors retry
dist-x86_64-mingw failed mysteriously without a logfile after 1 hour.

[bors]

⌛ Testing commit 342aa69 with merge b02a619...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing b02a619 to master...