This commit "inverts" the order of generic parameters/arguments of an
item and its parent. This is to fulfill chalk's expectation on the
order of `Substitution` for generic associated types and it's one step
forward for their support (hopefully).
Although chalk doesn't put any constraint on the order of `Substitution`
for other items, it feels natural to get everything aligned rather than
special casing GATs.
One complication is that `TyBuilder` now demands its users to pass in
parent's `Substitution` upon construction unless it's obvious that the
the item has no parent (e.g. an ADT never has parent). All users
*should* already know the parent of the item in question, and without
this, it cannot be easily reasoned about whether we're pushing the
argument for the item or for its parent.
Quick comparison of how this commit changes `Substitution`:
```rust
trait Trait<TP, const CP: usize> {
type Type<TC, const CC: usize> = ();
fn f<TC, const CC: usize>() {}
}
```
- before this commit: `[Self, TP, CP, TC, CC]` for each trait item
- after this commit: `[TC, CC, Self, TP, CP]` for each trait item
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