Prepare StreamSubject for adding Truth.assertThat(Stream).

[copybara-service]

Prepare StreamSubject for adding Truth.assertThat(Stream).

At that point, some calls to assertThat(Object) will become calls to the new overload. That would be a problem if:

• they call isEqualTo or isNotEqualTo, which we've turned into throwing @DoNotCall methods
• they have already collected the Stream or they want to operate on the Stream afterward

This CL makes isEqualTo and isNotEqualTo "work" (though it leaves them deprecated, since they're still a bad idea), and it avoids collecting the Stream until necessary.

The isEqualTo change requires some weird plumbing because I made its failure message retain our warning about Stream.equals. That requires creating a new StreamSubject with a different FailureMetadata instance, which isn't the kind of thing we normally do. (We've done something similar in ThrowableSubject, but it is simpler because (a) ThrowableSubject uses the "normal" (i.e.., non-no-arg) check and (b) ThrowableSubject doesn't have to worry about avoiding re-collecting a Stream.)

We may want to clamp back down on isEqualTo in the future. I've set myself a calendar reminder for mid-year. For now, I just want to make assertThat(Stream), which will already be mildly disruptive, from being even more disruptive.

(progress toward #746)

RELNOTES=Made StreamSubject avoid collecting the Stream until necessary, and made its isEqualTo and isNotEqualTo methods no longer always throw.