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

[pthariensflame]

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 Tracking Issue for Exclusive #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 impls const as possible; this involved making the existing impl Default no longer derived.
• Adding an impl for (const) Iterator, which felt in line with the existing impls for Future and Coroutine.
• Adding impls 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 Example for std::sync::Exclusive is dubious #146245.

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[joboet]

This needs to be bounded on Sync – size_hint takes &self.

[pthariensflame]

We figured this was similar to the case with Debug, which doesn’t need Sync despite taking &self because it doesn’t inspect self in any way. The default definition for size_hint is used here (so giving no information about the underlying iterator) instead of delegating through (which would require T: Sync).