Define `fn [_]::try_split_at(&self, usize) -> Option<(&Self, &Self)>`

[strake]

PR #54887

[Ben-Lichtman]

Adding the comment I made on the PR:

I know that this issue is long-closed, however I'd like to add that this would allow the removal of panicking code paths from critical code which may not panic, so I suggest a reconsideration.

Without this, one has to rely on unstable optimizations to remove panicking code.

[shumpohl]

This was stabilized in 1.80 under the name split_at_checked and split_at_mut_checked.

Issue: #119128
Stabilization: #124678