Rollup merge of #58034 - faern:stabilize-time-checked-add, r=alexcrichton
Rollup merge of #58034 - faern:stabilize-time-checked-add, r=alexcrichton

Stabilize the time_checked_add feature

Closes #55940

Stabilizes `checked_add` and `checked_sub` on `Instant` and `SystemTime`.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust