Rollup of 17 pull requests

Successful merges:

 - rust-lang#73943 (Document the unsafe keyword)
 - rust-lang#74062 (deny(unsafe_op_in_unsafe_fn) in libstd/ffi/c_str.rs)
 - rust-lang#74185 (Remove liballoc unneeded explicit link)
 - rust-lang#74192 (Improve documentation on process::Child.std* fields)
 - rust-lang#74409 (Change Debug impl of SocketAddr and IpAddr to match their Display output)
 - rust-lang#75195 (BTreeMap: purge innocent use of into_kv_mut)
 - rust-lang#75214 (Use intra-doc links in `mem::manually_drop` & `mem::maybe_uninit`)
 - rust-lang#75432 (Switch to intra-doc links in `std::process`)
 - rust-lang#75482 (Clean up E0752 explanation)
 - rust-lang#75501 (Move to intra doc links in std::ffi)
 - rust-lang#75509 (Tweak suggestion for `this` -> `self`)
 - rust-lang#75511 (Do not emit E0228 when it is implied by E0106)
 - rust-lang#75515 (Bump std's libc version to 0.2.74)
 - rust-lang#75517 (Promotion and const interning comments)
 - rust-lang#75519 (BTreeMap: refactor splitpoint and move testing over to unit test)
 - rust-lang#75530 (Switch to intra-doc links in os/raw/*.md)
 - rust-lang#75531 (Migrate unit tests of btree collections to their native breeding ground)

Failed merges:

r? @ghost

…crum

 BTreeMap: introduce marker::ValMut and reserve Mut for unique access

The mutable BTreeMap iterators (apart from `DrainFilter`) are double-ended, meaning they have to rely on a front and a back handle that each represent a reference into the tree. Reserve a type category `marker::ValMut` for them, so that we guarantee that they cannot reach operations on handles with borrow type `marker::Mut`and that these operations can assume unique access to the tree.

Including rust-lang#75195, benchmarks report no genuine change:
```
benchcmp old new --threshold 5
 name                                 old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 btree::map::iter_100                 3,333        3,023                -310   -9.30%   x 1.10
 btree::map::range_unbounded_vs_iter  36,624       31,569             -5,055  -13.80%   x 1.16
```

r? @Mark-Simulacrum

…crum

 BTreeMap: introduce marker::ValMut and reserve Mut for unique access

The mutable BTreeMap iterators (apart from `DrainFilter`) are double-ended, meaning they have to rely on a front and a back handle that each represent a reference into the tree. Reserve a type category `marker::ValMut` for them, so that we guarantee that they cannot reach operations on handles with borrow type `marker::Mut`and that these operations can assume unique access to the tree.

Including rust-lang#75195, benchmarks report no genuine change:
```
benchcmp old new --threshold 5
 name                                 old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 btree::map::iter_100                 3,333        3,023                -310   -9.30%   x 1.10
 btree::map::range_unbounded_vs_iter  36,624       31,569             -5,055  -13.80%   x 1.16
```

r? @Mark-Simulacrum

 BTreeMap: introduce marker::ValMut and reserve Mut for unique access

The mutable BTreeMap iterators (apart from `DrainFilter`) are double-ended, meaning they have to rely on a front and a back handle that each represent a reference into the tree. Reserve a type category `marker::ValMut` for them, so that we guarantee that they cannot reach operations on handles with borrow type `marker::Mut`and that these operations can assume unique access to the tree.

Including rust-lang#75195, benchmarks report no genuine change:
```
benchcmp old new --threshold 5
 name                                 old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 btree::map::iter_100                 3,333        3,023                -310   -9.30%   x 1.10
 btree::map::range_unbounded_vs_iter  36,624       31,569             -5,055  -13.80%   x 1.16
```

r? @Mark-Simulacrum