This commit also makes fields of `Take` private. I have no idea why they
were `pub(super)` before.

Some UI tests started failing after moving iterator adapters to different modules.

Yes, the order of evaluation *does* change depending on the types of
the operands. Cursed, I know.

I've elected to place this test into `expr/compound-assignment` creating
both the `expr` directory and the `compound-assignment` directory. I
plan in a future PR to also move the `if` directory and the loose `if`
tests into `expr/if` and other similar cleanups of the `test/ui`
directory.

Future work: Test more than just `+=`, but all operators. I don't know
if using a macro to generate these tests cases would be okay or not,
but it'd be boilerplatey without it. I'm also confident you cannot
change the evaluation order of one operator without changing all of
them.

Future work: Additionally, test more than just `i32 += i32` for the
primitive version. I don't actually know the full set of primitive
implementations, but I imagine there's enough to cause a combinatorial
explosion with the previous future work item. Somewhere on the order of
one to two hundred individual functions.

Stabilizes rust-lang#67521. In particular the following stable methods are stabilized as const fn:

* size
* align
* from_size_align

Add f{32,64}::is_subnormal

The docs recommend that you use dedicated methods instead of calling `classify` directly, although there isn't actually a way of checking if a number is subnormal without calling classify. There are dedicated methods for all other forms, excluding `is_zero` (which is just `== 0.0` anyway).

…=m-ou-se

Split each iterator adapter and source into individual modules

This PR creates individual modules for each iterator adapter and iterator source.

This is done to enhance the readability of corresponding modules (`adapters/mod.rs` and `sources.rs`) which were hard to navigate and read because of lots of repeated lines (e.g.: `adapters/mod.rs` was 3k lines long). This is also in line with some adapters which already had their own modules (`Flatten`, `FlatMap`, `Chain`, `Zip`, `Fuse`).

This PR also makes `Take`s adapter fields private (I have no idea why they were `pub(super)` before).

r? ``@LukasKalbertodt``

Stabilize alloc::Layout const functions

Stabilizes rust-lang#67521. In particular the following stable methods are stabilized as `const fn`:

* `size`
* `align`
* `from_size_align`

Stabilizing `size` and `align` should not be controversial as they are simple (usize and NonZeroUsize) fields and I don't think there's any reason to make them not const compatible in the future. That being true, the other methods are trivially `const`. The only other issue being returning a `Result` from a `const fn` but this has been made more usable by recent stabilizations.

…r=m-ou-se

Stabilize refcell_take

Tracking Issue: rust-lang#71395

``@KodrAus`` nominated this for FCP, so here's a PR!
I've never made a stabilization PR, so please mention if there's anything I can improve, thanks.

Clean up `StructuralEq` docs

…mulacrum

BTreeMap: address namespace conflicts

Fix an annoyance popping up whenever synchronizing the test cases with a version capable of miri-track-raw-pointers.

r? `@Mark-Simulacrum`

…gn, r=Mark-Simulacrum

Add test for eval order for a+=b

Yes, the order of evaluation *does* change depending on the types of
the operands. Cursed, I know.

I've elected to place this test into `expr/compound-assignment` creating
both the `expr` directory and the `compound-assignment` directory. I
plan in a future PR to also move the `if` directory and the loose `if`
tests into `expr/if` and other similar cleanups of the `test/ui`
directory.

Future work: Test more than just `+=`, but all operators. I don't know
if using a macro to generate these tests cases would be okay or not,
but it'd be boilerplatey without it. I'm also confident you cannot
change the evaluation order of one operator without changing all of
them.

Future work: Additionally, test more than just `i32 += i32` for the
primitive version. I don't actually know the full set of primitive
implementations, but I imagine there's enough to cause a combinatorial
explosion with the previous future work item. Somewhere on the order of
one to two hundred individual functions.

…ulacrum

BTreeMap: fix minor testing mistakes in rust-lang#78903

Mostly a duplicate test case
r? `@Mark-Simulacrum`

…mulacrum

BTreeMap: swap the names of NodeRef::new and Root::new_leaf

rust-lang#78104 preserved the name of Root::new_leaf to minimize changes, but the resulting names are confusing.

r? `@Mark-Simulacrum`

Stabilise `then`

Stabilises the lazy variant of rust-lang#64260 now that the FCP [has ended](rust-lang#64260 (comment)).

I've kept the original feature gate `bool_to_option` for the strict variant (`then_some`), and created a new insta-stable feature gate `lazy_bool_to_option` for `then`.