Rollup merge of #75195 - ssomers:btree_split_up_into_kv_mut, r=Mark-Simulacrum

BTreeMap: purge innocent use of into_kv_mut

Replace the use of `into_kv_mut` into more precise calls. This makes more sense if you know that the single remaining use of `into_kv_mut` is in fact evil and can be trialled in court (#75200) and sent to a correction facility (#73971).

No real performance difference reported (but functions that might benefit a tiny constant bit like `BTreeMap::get_mut` aren't benchmarked):
```
benchcmp old new --threshold 5
 name                       old ns/iter  new ns/iter  diff ns/iter  diff %  speedup
 btree::map::clone_fat_100  63,073       59,256             -3,817  -6.05%   x 1.06
 btree::map::iter_100       3,514        3,235                -279  -7.94%   x 1.09
```

0 files changed, 0 insertions, 0 deletions