Add Queue.imap, Topic.imap and Signal.map

[OlivierBlanvillain]

This PR adds imap/map on fs2.async.mutable.{Queue,Signal,Topic}. The follow up would be to define the corresponding Invariant instances in fs2-cats/fs2-scalaz.

I'm not sure if I should test that with the invariant laws, bringing the ones from cats with an additional % "test" dep, or if you consider the type checker to be enough in that case...

functional-streams-for-scala/fs2-cats#6

[mpilquist]

👍 Can we add imap to mutable.Signal too?

[OlivierBlanvillain]

Like this?

[OlivierBlanvillain]

Ah didn't even notice they where two Signals 😄. Actually, the immutable version already has ImmutableSignalSyntax which give it a map, so I reverted my changes in that file.

(Note that because of inheritance it would be weird to have map define on immutable.Signal, as it would expose both map and imap on the mutable version with different return types.)