Auto merge of #142885 - a1phyr:borrowed_cursor_to_buf, r=Mark-Simulacrum

core: Add `BorrowedCursor::with_unfilled_buf`

Implementation of https://github.com/rust-lang/libs-team/issues/367.

This mainly adds `BorrowedCursor::with_unfilled_buf`, with enables using the unfilled part of a cursor as a `BorrowedBuf`.

Note that unlike the ACP, `BorrowedCursor::unfilled_buf` was moved to a `From` conversion. This is more consistent with other ways of creating a `BorrowedBuf` and hides a bit this conversion that requires unsafe code to be used correctly.

Cc rust-lang/rust#78485 rust-lang/rust#117693