Rollup merge of #153038 - pthariensflame:syncview-rename, r=Amanieu

core::sync: rename `Exclusive` to `SyncView` and make improvements

This PR implements the renaming of `core::sync::Exclusive` to `SyncView` as decided in #98407.  To preserve the ability to search for the old name, it adds `Exclusive` as a `doc_alias`.

It also makes the following additional changes:

- Converting the `get_mut` method to being an instance of `AsMut::as_mut`.  In the process, it makes both the new `impl AsMut` and the existing `impl AsRef` `const`, and it also renames `get_pin_mut` to `as_pin_mut` for consistency.  This direction follows a suggestion from #98407.
- Adding an `as_pin` method that can only be used when the wrapped type implements `Sync`, to complete the square of access methods.
- Making as many of the existing `impl`s `const` as possible; this involved making the existing `impl Default` no longer derived.
- Adding `impl`s for `AsyncFnOnce`, `AsyncFnMut`, and `AsyncFn`, akin to the existing `impls` for `FnOnce`, `FnMut`, and `Fn`.

It does not yet do the following, which may be desirable:

- Fixing/improving the documentation to address the concern pointed out in #146245.

It previously did the following, but this was removed after discussion:
- Adding an `impl` for (`const`) `Iterator`, which felt in line with the existing `impl`s for `Future` and `Coroutine`.

Author: jhpratt

library/core/src/sync/exclusive.rs

@@ -3,6 +3,6 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 pub mod atomic;
-mod exclusive;
+mod sync_view;
 #[unstable(feature = "exclusive_wrapper", issue = "98407")]
-pub use exclusive::Exclusive;
+pub use sync_view::SyncView;