Slice reads the next length bytes of r into a slice.
This slice may be backed by mmap'ed memory. Currently, this memory
will never be unmapped. The second result reports whether the
backing memory is read-only.
SliceRO returns a slice containing the next length bytes of r
backed by a read-only mmap'd data. If the mmap cannot be
established (limit exceeded, region too small, etc) a nil slice
will be returned. If mmap succeeds, it will never be unmapped.