RFC: unexpanders take priority

[dselsam]

This PR addresses an issue raised by @dwrensha at https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/controlling.20delaboration. Currently, unexpanders for structures are ignored since they are tried after the builtin structure unexpanders. Example:

structure Foo where
  x : Nat
  y : Nat

syntax term " ♬ " term : term

macro_rules
| `( $a ♬ $b ) => `(Foo.mk $a $b)

#check 1 ♬ 2 -- { x := 1, y := 2 }

@[appUnexpander Foo] def unexpandFoo : Lean.PrettyPrinter.Unexpander
  | `({ x:= $a, y:= $b }) => `($a ♬ $b)
  | _ => throw ()

#check 3 ♬ 4 -- { x := 3, y := 4 } 

At @Kha's suggestion, this PR avoids the need to cache calls within the delaborator at the cost of requiring exponential time on an obscure edge case in which a stack of unexpanders all fail. The issue is documented in the code in case there is interest in addressing it in the future.