FEATURE: multi-shape parallel matching, out-of-order binding constraint solutions.

Arthur (from burn discord) asks:
```rust
assert_shapes!(
    means => B 3,
    rotations => B 4,
    img => N H W,
    depths => N H W,
);
```

There are two issues here:
 * making the solver multi (contract, shape)
 * expanding the solver so that an expression can have more than one unbound value, provided that the system of expressions is fully solvable.