BytesCursor abstraction in FuseDevWriter

[stepancheg]

FuseDevWriter.buf field is ManuallyDrop<Vec<u8>>, but actually it is a pair of &mut [u8] space and position.

Express this logic more explicitly with

pub(crate) struct BytesCursor<'a> {
    slice: &'a mut [u8],
    position: usize,
}

Because:

• less unsafe code
• more operations are checked
• with this change it is clear that there's no UB (for example, related to aliasing &mut pointers). It may be not an issue, but with this change is much more clear