This PR adds proper support for `let_fun` in `simp`.

This PR adds the following features to `simp`:
- A routine for simplifying `let` telescopes, like what leanprover#6220 did for `letFun` telescopes. Furthermore, simp converts `letFun`s into `have`s (nondependent lets), and we remove the leanprover#6220 routine.
- A `+letToHave` configuration option (enabled by default) that converts lets into haves when possible, when `-zeta` is set. Previosuly Lean would need to do a full typecheck the bodies of `let`s, but the `letToHave` procedure can be faster, and it modifies the `let`s in an entire expression at once.
- A `+zetaHave` configuration option, to turn off zeta reduction of `have`s specifically. The motivation is that dependent `let`s can only be dsimped by let, so zeta reducing just nondependent lets is a reasonable way to make progress.

This PR adds the following features to `simp`:
- A routine for simplifying `let` telescopes, like what leanprover#6220 did for `letFun` telescopes. Furthermore, simp converts `letFun`s into `have`s (nondependent lets), and we remove the leanprover#6220 routine.
- A `+letToHave` configuration option (enabled by default) that converts lets into haves when possible, when `-zeta` is set. Previosuly Lean would need to do a full typecheck the bodies of `let`s, but the `letToHave` procedure can be faster, and it modifies the `let`s in an entire expression at once.
- A `+zetaHave` configuration option, to turn off zeta reduction of `have`s specifically. The motivation is that dependent `let`s can only be dsimped by let, so zeta reducing just nondependent lets is a reasonable way to make progress.

This PR adds the following features to `simp`:
- A routine for simplifying `let` telescopes, like what leanprover#6220 did for `letFun` telescopes. Furthermore, simp converts `letFun`s into `have`s (nondependent lets), and we remove the leanprover#6220 routine.
- A `+letToHave` configuration option (enabled by default) that converts lets into haves when possible, when `-zeta` is set. Previosuly Lean would need to do a full typecheck the bodies of `let`s, but the `letToHave` procedure can be faster, and it modifies the `let`s in an entire expression at once.
- A `+zetaHave` configuration option, to turn off zeta reduction of `have`s specifically. The motivation is that dependent `let`s can only be dsimped by let, so zeta reducing just nondependent lets is a reasonable way to make progress.

This PR adds the following features to `simp`:
- A routine for simplifying `have` telescopes in a way that avoids
quadratic complexity arising from locally nameless expression
representations, like what #6220 did for `letFun` telescopes.
Furthermore, simp converts `letFun`s into `have`s (nondependent lets),
and we remove the #6220 routine since we are moving away from `letFun`
encodings of nondependent lets.
- A `+letToHave` configuration option (enabled by default) that converts
lets into haves when possible, when `-zeta` is set. Previously Lean
would need to do a full typecheck of the bodies of `let`s, but the
`letToHave` procedure can skip checking some subexpressions, and it
modifies the `let`s in an entire expression at once rather than one at a
time.
- A `+zetaHave` configuration option, to turn off zeta reduction of
`have`s specifically. The motivation is that dependent `let`s can only
be dsimped by let, so zeta reducing just the dependent lets is a
reasonable way to make progress. The `+zetaHave` option is also added to
the meta configuration.
- When `simp` is zeta reducing, it now uses an algorithm that avoids
complexity quadratic in the depth of the let telescope.
- Additionally, the zeta reduction routines in `simp`, `whnf`, and
`isDefEq` now all are consistent with how they apply the `zeta`,
`zetaHave`, and `zetaUnused` configurations.

The `letToFun` option is addressing a TODO in `getSimpLetCase` ("handle
a block of nested let decls in a single pass if this becomes a
performance problem").

Performance should be compared to before #8804, which temporarily
disabled the #6220 optimizations for `letFun` telescopes.

Good kernel performance depends on carefully handling the `have`
encoding. Due to the way the kernel instantiates bvars (it does *not*
beta reduce when instantiating), we cannot use congruence theorems of
the form `(have x := v; f x) = (have x ;= v'; f' x)`, since the bodies
of the `have`s will not be syntactically equal, which triggers zeta
reduction in the kernel in `is_def_eq`. Instead, we work with `f v = f'
v'`, where `f` and `f'` are lambda expressions. There is still zeta
reduction, but only when converting between these two forms at the
outset of the generated proof.