The semantics was weird. It seems Agda is also having problems with
it. Here is an example that demonstrates how weird the semantics is:
```lean
check (fun {β α} (a : α) (b : β) => (b, a) : {α : Type} → {β : Type} → (a : α) → (b : β) → β × α)
-- Same example using `def`
def f : {α : Type} → {β : Type} → α → β → β × α :=
fun {β : Type} {α : Type} (a : α) (b : β) => (b, a)
```
Both commands were being accepted before this commit. Note that it
flips `β` and `α`.
Here is an example that did not work before this commit and would
confuse users.
```lean
check
let id := fun {α} (a : α) => a;
id [id 1]
```
users would have to write
```lean
check
let id {α} (a : α) := a;
id [id 1]
```
@Kha The Delaborator.lean test broke and I "fixed" by removing the
`{}` from it, and copying `produced` over `expected`. Please make sure
it still makes sense.
