Follow up to the previous commit refactoring `core::num` by moving the
user-facing error types and trait implementations back out of `imp` and
into a new module under `core::num`.

Also updates the marker_impls macro docs to use PointeeSized bound, as most uses of the macro now do.

Split binarycat's entry into multiple lines, one for each extra email

Main changes:
- Inline `encode_query_cache` and `TyCtxt::serialize_query_result_cache`
- Pull value promotion out of `OnDiskCache::drop_serialized_data`
- Panic if `on_disk_cache` is None in an incremental-only path

…Simulacrum

std: use `OnceLock` for Xous environment variables

There's no need for exposed-provenance-shenanigans here...

CC @xobs

…ointee, r=Mark-Simulacrum

Update `UnsafeUnpin` impls involving extern types.

`UnsafeUnpin` tracking issue: rust-lang#125735

Relaxes from `T: ?Sized` (i.e. `T: MetaSized`) to `T: PointeeSized` for the `UnsafeUnpin` impls for pointers, references, and `PhantomData<T>`, and for the negative `UnsafeUnpin` impl for `UnsafePinned<T>`. (Compare to the impls for `Freeze` on lines 911-921.)

Both `UnsafeUnpin` and `extern type`s (the only way to have a `!MetaSized` type) are unstable, so this should have no effect on stable code.

Also updates the marker_impls macro docs to use PointeeSized bound, as most uses of the macro now do.

Concretely, this change means that the following types will newly implement `UnsafeUnpin`:

* pointers and references to `T` where `T` is an `extern type`
* `PhantomData<T>` where `T` is an extern type
* either of the above where `T` is a `struct` or tuple with `extern type` tail

Additionally, the negative `UnsafeUnpin` impl for `UnsafePinned<T>` is also relaxed to `T: PointeeSized` to align with the analogous negative `Freeze` impl for `UnsafeCell<T>`, even though both structs have `T: ?Sized` in their declaration (which probably should be relaxed, but that is a separate issue), so this part of the change doesn't actually *do* anything currently, but if `UnsafeCell` and `UnsafePinned` are later relaxed to `T: PointeeSized`, then the negative impl will apply to the newly possible instantiations. Also cc rust-lang#152645 that these impls compile at all.

Inline and simplify some code for saving incremental data to disk

Main changes:
- Inline `encode_query_cache` and `TyCtxt::serialize_query_result_cache`
- Pull value promotion out of `OnDiskCache::drop_serialized_data`
- Panic if `on_disk_cache` is None in an incremental-only path

…Simulacrum

num: Separate public API from internal implementations

Currently we have a single `core::num` module that contains both thin wrapper API and higher-complexity numeric routines. Restructure this by moving implementation details to a new `imp` module.

This results in a more clean separation of what is actually user-facing compared to items that have a stability attribute because they are public for testing.

The first commit does the actual change then the second moves a portion back.

…rycat

.mailmap: fix broken line with multiple emails

Split binarycat's entry into multiple lines, one for each extra email

…bi1kenobi,GuillaumeGomez

rustdoc-json: fix incorrect documentation for VariantKind::Struct

Seems to have been copy-and-pasted from `Enum::variants`, but `VariantKind::Struct::fields` is for struct variant fields, not enum variants.