This PR finishes post-stage0-cleanup after #8914 and #8929. Also:
- adds configuration options for `haveI` and `letI` terms.
- adds `letConfig` parser alias
This PR modifies `let` and `have` term syntaxes to be consistent with
each other. Adds configuration options; for example, `have` is
equivalent to `let +nondep`, for *nondependent* lets. Other options
include `+usedOnly` (for `let_tmp`), `+zeta` (for `letI`/`haveI`), and
`+postponeValue` (for `let_delayed)`. There is also `let (eq := h) x :=
v; b` for introducing `h : x = v` when elaborating `b`. The `eq` option
works for pattern matching as well, for example `let (eq := h) (x, y) :=
p; b`.
Future PRs will add these options to tactic syntax, once a stage0 update
has been done.
@Kha The hack I posted at Zulip didn't really work
```
macro x:ident : tactic => throw $ Lean.Macro.Exception.error x s!"unknown tactic '{x.getId}'"
```
For example, we would still get a weird error message at
```
theorem ex3 (x : Nat) : x = x → x = x :=
have x = x by foo (aaa bbb) -- The error would be at `bbb`
fun h => h
```
There were other minor issues that could be fixed, but this one was bad.
