Inconsistency between `BytesMut` and `Vec<u8>` value for `BufMut::remaining_mut`

[paolobarbolini]

The BufMut::remaining_mut implementation for BytesMut does usize::MAX - self.len(), while the Vec<u8> impl does isize::MAX as usize - self.len() instead. Shouldn't they both use isize::MAX given than BytesMut is represented as a Vec<u8> underneath?