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#78485rust-lang/rust#117693
This enable removing the `start` field, so `BorrowedCursor` fits in a
single register. Because `written` is almost always used in difference
with another call, this changes nothing else in practice.
I assume that this method was there for completeness, but there is
hardly any useful use of it: the buffer it gave was not always connected
to the start of the cursor and its use required `unsafe` anyway to mark
the bytes as initialized.
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.
