Allow for nested chex.chexify

Hello, I have a dilemma with chexify - consider the following code:
```
# If this is not commented out, the second test will fail
# If this is commented out, the first test will fail
@chex.chexify
@jax.jit
def log_safe(x: jnp.array) -> jnp.array:
chex.assert_trees_all_equal(x > 0, jnp.ones_like(x, dtype=bool))
return jnp.log(x)

@chex.chexify
@jax.jit
def combo_safe(x: jnp.array) -> jnp.array:
chex.assert_trees_all_equal(x != 1, jnp.ones_like(x, dtype=bool))
return log_safe(x) / (x - 1)

def test_log_safe() -> None:
x = jnp.array([1.0, 2.0, 3.0, -1.0])
with pytest.raises(Exception):
log_safe(x)
log_safe.wait_checks()

x = jnp.array([1.0, 2.0, 3.0, 4.0])
assert jnp.array_equal(log_safe(x), jnp.log(x))
log_safe.wait_checks()

def test_combo_safe() -> None:
x = jnp.array([1.0, 2.0, 3.0, 4.0])
with pytest.raises(Exception):
combo_safe(x)
combo_safe.wait_checks()

x = jnp.array([2.0, 3.0, 4.0, 5.0])
assert jnp.array_equal(combo_safe(x), jnp.log(x) / (x - 1))
combo_safe.wait_checks()
```

If I comment out the first chexify the `test_log_safe` test will fail with `RuntimeError: Value assertions can only be called from functions wrapped with @chex.chexify. See the docs.` which makes sense to me. However, once I add the decorator back in, the second test fails with `RuntimeError: Nested @chexify wrapping is disallowed. Make sure that you only wrap the function at the outermost level.`

A hack in this simple scenario would be to make two versions of the function, a `log_safe` without the chexify decorator and a `log_safe_test = chex.chexify(log_safe)` and only call the `log_safe_test` version during my test. However, that solution is pretty clumsy, especially if I have a lot of these scenarios. In a codebase that is fully end-to-end jax, that would mean all but the outermost function require this hack. Would it be possible to allow for nested chex.chexify where subsequent applications of the macro simply do nothing, or just raise a warning?