Rollup merge of #143829 - a1phyr:trim_borrowed_buf, r=ChrisDenton

Trim `BorrowedCursor` API

This PR removes some method from the unstable `BorrowedCursor` type. A rational for each change can be found in the message of each commit.

I don't think that an ACP is required for this, please tell me if it is not the case.

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

Author: fmease