feat: generalize elab_as_elim to allow arbitrary motive applications
#5510
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Simplifies the elab-as-elim procedure, and as a consequence now eliminators can include arbitrary expressions.
For example, the following is now accepted. It will generalize `Int.natAbs _` from the expected type.
```lean
@[elab_as_elim]
theorem natAbs_elim {motive : Nat → Prop} (i : Int)
(hpos : ∀ (n : Nat), i = n → motive n)
(hneg : ∀ (n : Nat), i = -↑n → motive n) :
motive (Int.natAbs i) := by sorry
```
Closes leanprover#4086
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Now the elab-as-elim procedure allows eliminators whose result is an arbitrary application of the motive. For example, the following is now accepted. It will generalize
Int.natAbs _from the expected type.This change simplifies the elaborator, since it no longer needs to keep track of discriminants (which can easily be read off from the return type of the eliminator) or the difference between "targets" and "extra arguments" (which are now both "major arguments" that should be eagerly elaborated).
Closes #4086