Changes to the universe mechanism

[breuleux]

• The universe argument is moved to the end of the parameter list, rather than the beginning, in order to be compatible with partial application.
• The handle primitive becomes make_handle and is changed.
  • It does not set an initial value in the universe. universe_setitem must be called to do so.
  • make_handle takes a universe and a type and returns a new universe and a handle, which allows us to order calls to make_handle and guarantee that CSE cannot merge similar handles.
  • A graph transform is applied in the Relay backend so that it matches the behavior of RefCreate.
• The make_cell composite behaves like handle used to.
• The handle_get and handle_set composites are renamed to cell_get and cell_set.
• The feature works in more situations, and more tests are added.

[abergeron]

The only question I have is why not have make_cell as the primitive? Otherwise I'm ok.

[breuleux]

The only question I have is why not have make_cell as the primitive?

Essentially, because it is simpler for optimization purposes if only one primitive sets things in the universe. For example, we can simplify getitem(setitem(U, h, v), h) to v, but if make_cell was a primitive we would also need to consider U2, h = make_cell(U1, v); getitem(U2, h), and so on.