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.