Simplify outgoing notifications, remove NoParams

[Xanewok]

Work towards #602. Since I'll want use different notifications (window/showMessage) than the currently hardcoded PublishDiagnostics and rustDocument/* ones, I tried to generalize the Output::notify, which is now easily possibly thanks to Action trait (thanks @alexheretic!).
The trait is generic enough (provides method and params type) to be easily reused, but it's not as clean as it could be - per documentation implementers are messages handled by the server (so client->server), whereas I added implementation of the ones sent to the client instead.

Ideally this should be solved using gluon-lang/lsp-types#19 (which I didn't do yet... 😢 ).

Additionally replaced NoParams with (), as these serve the same purpose and not to introduce another custom type.

r? @nrc

[nrc]

First two commits look good, I'm not sure about removing NoParams - did this work OK?

[nrc]

The point in having NoParams is for the serialisation and deserialisation, I think that if you use () there are bugs both ways (serialised as () rather than {} or maybe ``, iirc). It's possible some other change means this is no longer an issue though.

[Xanewok]

I will be completely honest here - I'm guilty of relying solely on test suite for NoParams change. Now that I look at it they're not covering much messages with NoParams, except for test_shutdown, but that hands off the already structured Request.
I may have jumped gun on that one. I'll discard the last commit for now, thanks for double-checking this!

[Xanewok]

Rebased and force-pushed without the last NoParams -> () commit. As per #603 (comment), I'll revisit the last change when I have more tests.

[nrc]

Thanks!