This PR adds an alternative implementation of `DerivingBEq` based on
comparing `.ctorIdx` and using a dedicated matcher for comparing same
constructors (added in #10152), to avoid the quadratic overhead of the
default match implementation. The new option
`deriving.beq.linear_construction_threshold` sets the constructor count
threshold (10 by default) for using the new construction. Such instances
also allow `deriving ReflBEq, LawfulBeq`, although these proofs for
these properties are still quadratic.
This PR introduces the `@[specs]` attribute. It can be applied to
(certain) type class instances and define “specification theorems” for
the class’ operations, by taking the equational theorems of the
implementation function mentioned in the type class instance and
rephrasing them in terms of the overloaded operations. Fixes#5295.
Example:
```
inductive L α where
| nil : L α
| cons : α → L α → L α
def L.beqImpl [BEq α] : L α → L α → Bool
| nil, nil => true
| cons x xs, cons y ys => x == y && L.beqImpl xs ys
| _, _ => false
@[method_specs] instance [BEq α] : BEq (L α) := ⟨L.beqImpl⟩
/--
info: theorem instBEqL.beq_spec_2.{u_1} : ∀ {α : Type u_1} [inst : BEq α] (x_2 : α) (xs : L α) (y : α) (ys : L α),
(L.cons x_2 xs == L.cons y ys) = (x_2 == y && xs == ys)
-/
#guard_msgs(pass trace, all) in
#print sig instBEqL.beq_spec_2
```
It also introduces the `method_specs_norm` simpset to allow registering
further normalization of the theorems. The intended use of this is to
rewrite, say, `Append.append` to the `HAppend.hAppend` (i.e. `++`) that
the user wants to see. Library annotations to follow in a separate PR.
