Add a convenient shorthand for predicating effects on a boolean signal.

[mn98]

Purely for convenience, this allows for writing
signal.predicate(f)
instead of
signal.get.flatMap(f.whenA)

[mpilquist]

Suggested change

	def predicate(f: F[_])(implicit F: Applicative[F] with FlatMap[F]): F[Unit] =
	def predicate(f: F[_]: Monad): F[Unit] =

[mn98]

Hmm, that form doesn't compile for me, but this form does:
def predicate(f: F[_])(implicit F: Monad[F]): F[Unit]
Is that ok?

[mpilquist]

Whoops, yep

[mn98]

Great; committed.

[mn98]

I would add that although this is a minor convenience feature it's probably still worth merging. I get the impression much of the *Ops sugar will eventually move out to a separate module/project, which would then include this little gem. I probably wouldn't have raised this PR if BooleanSignalOps didn't already exist.