Better error message for missing implicits

[non]

Lots of newcomers forget to import implicit instances (e.g. cats.implicits._) which causes lots of pain.

It might be nice if when Functor[X] is not found, the the message indicates that you may have failed to import it (and can even mention cats.std.all._ or cats.implicits._).

What do you all think?

[non]

(The bad part here is that I think this would require annotating every type class with some kind of message. Either that, or wiring it up to Simulacrum somehow.)

[milessabin]

Big +1 for wiring up in Simulacrum.

[erwan]

I just did a PR in Simulacrum (with help from @larsrh) : typelevel/simulacrum#144