feat: `guard` and `check` in `grind_pattern` (#11428)

This PR implements support for **guards** in `grind_pattern`. The new
feature provides additional control over theorem instantiation. For
example, consider the following monotonicity theorem:

```lean
opaque f : Nat → Nat
theorem fMono : x ≤ y → f x ≤ f y := ...
```

We can use `grind_pattern` to instruct `grind` to instantiate the
theorem for every pair `f x` and `f y` occurring in the goal:

```lean
grind_pattern fMono => f x, f y
```

Then we can automatically prove the following simple example using
`grind`:

```lean
/--
trace: [grind.ematch.instance] fMono: f a ≤ b → f (f a) ≤ f b
[grind.ematch.instance] fMono: f a ≤ c → f (f a) ≤ f c
[grind.ematch.instance] fMono: f a ≤ a → f (f a) ≤ f a
[grind.ematch.instance] fMono: f a ≤ f (f a) → f (f a) ≤ f (f (f a))
[grind.ematch.instance] fMono: f a ≤ f a → f (f a) ≤ f (f a)
[grind.ematch.instance] fMono: f (f a) ≤ b → f (f (f a)) ≤ f b
[grind.ematch.instance] fMono: f (f a) ≤ c → f (f (f a)) ≤ f c
[grind.ematch.instance] fMono: f (f a) ≤ a → f (f (f a)) ≤ f a
[grind.ematch.instance] fMono: f (f a) ≤ f (f a) → f (f (f a)) ≤ f (f (f a))
[grind.ematch.instance] fMono: f (f a) ≤ f a → f (f (f a)) ≤ f (f a)
[grind.ematch.instance] fMono: a ≤ b → f a ≤ f b
[grind.ematch.instance] fMono: a ≤ c → f a ≤ f c
[grind.ematch.instance] fMono: a ≤ a → f a ≤ f a
[grind.ematch.instance] fMono: a ≤ f (f a) → f a ≤ f (f (f a))
[grind.ematch.instance] fMono: a ≤ f a → f a ≤ f (f a)
[grind.ematch.instance] fMono: c ≤ b → f c ≤ f b
[grind.ematch.instance] fMono: c ≤ c → f c ≤ f c
[grind.ematch.instance] fMono: c ≤ a → f c ≤ f a
[grind.ematch.instance] fMono: c ≤ f (f a) → f c ≤ f (f (f a))
[grind.ematch.instance] fMono: c ≤ f a → f c ≤ f (f a)
[grind.ematch.instance] fMono: b ≤ b → f b ≤ f b
[grind.ematch.instance] fMono: b ≤ c → f b ≤ f c
[grind.ematch.instance] fMono: b ≤ a → f b ≤ f a
[grind.ematch.instance] fMono: b ≤ f (f a) → f b ≤ f (f (f a))
[grind.ematch.instance] fMono: b ≤ f a → f b ≤ f (f a)
-/
#guard_msgs in
example : f b = f c → a ≤ f a → f (f a) ≤ f (f (f a)) := by
  set_option trace.grind.ematch.instance true in
  grind
```

However, many unnecessary theorem instantiations are generated.

With the new `guard` feature, we can instruct `grind` to instantiate the
theorem **only if** `x ≤ y` is already known to be true in the current
`grind` state:

```lean
grind_pattern fMono => f x, f y where
  guard x ≤ y
  x =/= y
```

If we run the example again, only three instances are generated:

```lean
/--
trace: [grind.ematch.instance] fMono: a ≤ f a → f a ≤ f (f a)
[grind.ematch.instance] fMono: f a ≤ f (f a) → f (f a) ≤ f (f (f a))
[grind.ematch.instance] fMono: a ≤ f (f a) → f a ≤ f (f (f a))
-/
#guard_msgs in
example : f b = f c → a ≤ f a → f (f a) ≤ f (f (f a)) := by
  set_option trace.grind.ematch.instance true in
  grind
```

Note that `guard` does **not** check whether the expression is
*implied*. It only checks whether the expression is *already known* to
be true in the current `grind` state. If this fact is eventually
learned, the theorem will be instantiated.

If you want `grind` to check whether the expression is implied, you
should use:

```lean
grind_pattern fMono => f x, f y where
  check x ≤ y
  x =/= y
```

Remark: we can use multiple `guard`/`check`s in a `grind_pattern`
command.