lazy: check sentinel identity with is, not ==

[wchargin]

Summary:
This is not likely to be a problem in practice, but we might as well do
the safe thing when comparing to a sentinel.

Test Plan:
Unit test added, for good measure.

wchargin-branch: lazy-sentinel-is

[nfelt]

It seems a little better to just have a counter within foo() itself? Since what we are really tracking is that foo() should be called just once, regardless of what it does. That way we separate concerns so that EqualToEverything is only doing what it says on the label and not also being a counter.

This does require an indirection to get around python read-only closures, e.g. foo.counter = getattr(foo, 'counter', 0) + 1.

[wchargin]

I originally had the standard count_box = [0] trick, which I can
reinstate. You can’t just store the value on foo itself because that
will refer to the wrapped LazyModule; i.e., using

    @lazy.lazy_load("foo")
    def foo():
      foo.counter = getattr(foo, "counter", 0) + 1
      return EqualToEverything()

raises “Circular import when resolving LazyModule 'foo'”. You can get around this with

    @lazy.lazy_load("foo")
    def foo():
      foo.counter += 1
      return EqualToEverything()
    foo.counter = 0

but given that it’s this subtle I’m inclined to stick to the much
simpler close-over-[0], which is at least somewhat idiomatic.

[nfelt]

Okay, I was being too clever. The boxing singleton list is fine, sure.