Code action: add constraint (haskell/ghcide#653)

* Add missing instance constraint

* Add missing instance constraint with existing constraints

* Add missing function constraint

* Add missing function consraint with existing constraints

* Add some comments

* Improve type signature regex

* Remove redundant bracket

* Improve missing constraint searching.

Create entrypoint for missing constraint code action, in order to have a more
efficient parsing by routing to the relevant implementation.

Fix type signature name parsing.

Minor refactor.

* Minor refactor