This PR fixes a bug that could cause the `injectivity` tactic to fail in
reducible mode, which could cause unfolding lemma generation to fail
(used by tactics such as `unfold`). In particular,
`Lean.Meta.isConstructorApp'?` was not aware that `n + 1` is equivalent
to `Nat.succ n`.
Closes#5064
`simp only` will not apply this simproc anymore. Users must now write
`simp only [reduceCtorEq]`. See RFC #5046 for motivation.
This PR also renames simproc to `reduceCtorEq`.
close#5046
@semorrison A few `simp only ...` tactics will probably break in
Mathlib. Fix: include `reduceCtorEq`.
closes#3022
With this commit, given the declaration
```
def foo : Nat → Nat
| 0 => 2
| n + 1 => foo n
```
when we unfold `foo (n+1)`, we now obtain `foo n` instead of `foo
(Nat.add n 0)`.
