Add a safe way to fill a ReadBuf

[notgull]

Closes #3509

[seanmonstar]

Thanks for the PR!

My initial thoughts were that we could provide a couple methods like in BorrowedCursor. Looking through them, it occurs to me that the only "basic" safe way to write data is to like append(&[u8]). Otherwise, even if you can safely access all initialized_mut(), it would still be unsafe to state an arbitrary number of bytes have been written.

[notgull]

My initial thoughts were that we could provide a couple methods like in BorrowedCursor.

Any specific methods?