Make it possible to write expect.that(optional).isPresent(), assertWithMessage(...).that(optional).isPresent(), etc., including for Stream as well as Optional.

[copybara-service]

Make it possible to write expect.that(optional).isPresent(), assertWithMessage(...).that(optional).isPresent(), etc., including for Stream as well as Optional.

That is, you no longer need to use about(optionals()) (or about(streams())). (Later, we'll do the same thing for lesser-used APIs like `optionalInts().)

This CL does not make it possible to write Truth.assertThat(optional).isPresent(): For that, you still need to use Truth8. A future CL will eliminate the need to use Truth8.

This continues our work on #746.

RELNOTES=Added that overloads to make it possible to write type-specific assertions when using expect.that(optional) and expect.that(stream).