[RFC] Implement Boolean-to-Prop coercion using Bool.asProp

[gebner]

A while ago, Sebastian suggested[1] that we should redefine the Boolean-to-Prop coercion using a dedicated function. This recently came up again[3] because we're porting material about List.filter, which is now defined on Booleans and thus exercises the coercion heavily.

def Bool.asProp (b : Bool) : Prop :=
  b = true

instance : Coe Bool Prop where
  coe := Bool.asProp

Pros:

1. No accidental simplification of ¬ b = true to b = false (which requires duplication of simp lemmas).
2. decide b = b is defeq (after Make Decidable a subtype of Bool #2038)
3. Pretty-printing and norm_cast work out-of-the-box.

Cons:

1. simp only [h] no longer works as well for a hypothesis h : b with b : Bool.

Original proposal: [1] https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/rfc.3A.20theorem.20names/near/240161960
Other threads:
[2] https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/BEq.20Subtype/near/296650727
[3] https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/Prop.20-.3E.20Bool.20regression/near/321996465

[gebner]

Additional proposal:

• Make simp [h] add show b = true from h to the simp set if h : Bool.asProp b.