GitLab

* refactor: move environment-based command restrictions to be stored with the command itself

* Staticcheck fixes

This fixes all staticcheck warnings for this library.

Only use custom encoders for values.

fixes #65

Also, remove SetEncoder. Always derive the correct encoder from the request.

to func(Response, ResponseEmitter) error

(and test)

Also, make it easier to actually use ParseBodyArguments. Now it only returns an
error when something actually goes wrong.

**Returning errors/calling:**

Instead of having a dance where we:
  * Close the response emitter immediately if the command succeeds.
  * Return an error, call SetError, and then close the response emitter if it
    fails.

Just set the error from within the `Call` command and always close the response
emitter.

**Doner**

Get rid of the Done channel/doner interface from the response emitter. We close
the response emitter after finishing the Call *anyways* so this channel isn't
necessary (anymore, at least).

We *always* expect environments to be able to return a context so we might as
well enforce this with the type system.