Make usage of _z_bytes_t / _z_slice_t more consistent

[sashacmc]

Describe the release item

If variable named payload it should be _z_bytes_t (or z_owned_bytes_t, z_loaned_bytes_t in API)
If at the level of abstraction under consideration we can forget about _z_bytes_t and we can honestly consider memory as a solid piece (aka _z_slice_t), then the variable name should not be payload to avoid confusion.
There should also be no variables with the name bytes and type _z_slice_t