Parametrize Editor by the Completer type

[Marwes]

This allows the Editor to implement Sync as long as it contains a completer which implement Sync. It lets the Editor take ownership of the completer.

Fixes #49

[Marwes]

Happy to update this PR if you find any problems with it. It may be a bit awkward that users need to explicitly parametrize Editor with the completer if it is unused but I am not sure what is the best way to avoid that.

[kkawakam]

Hey @Marwes, sorry that I didn't get back to you on your issue during the week. I agree that with you that it is a bit awkward with explicitly parametrizing Editor if there is no completer but I am okay with it. Can you update the README.md example to reflect that change?

I also just merged in another branch into master and it has caused some conflicts with your branch.

[Marwes]

No problem I am in no rush, just wanted to make sure making the change was possible.

A tip for keeping the README example updated, skeptic is really useful to ensure that the examples in the README always builds (though it can be a bit slow if you start having 10+ examples as I have found).

[Marwes]

👍

[kkawakam]

@kkawakam note to self: indicate in release notes this is a breaking change because of the need to explicitly parametrize the Editor when there is no completer.

[Marwes]

@kkawakam I used the pattern of () as a 'null object' in this PR which may not be the most intuitive solution. I used this pattern in gluon as well but I have been convinced to change it to an explicit type for the null object instead to increase discoverability (so maybe a pub struct NullCompleter; instead).

I obviously don't mind () myself but I thought I would let you know that it might not be a common pattern that is easily understandable.