New completion sorting approach

[Moosieus]

Addressing #643. This PR replaces the completion 'boost' function set_sort_scope and an enumerated set of options, based on relative "distance" (for lack of better term) to a user's locally opened module.

At time of writing, I'm definitely looking for feedback and further improvements before this is merged.

I've a few immediate things I'm considering for improvement:

• I chose the new naming conventions (ex: SortScope, set_sort_scope) on the spot. Perhaps there's better names?
• Local sorting is relatively ad-hoc. Should it be more formally defined?
• I wonder if Module/Behaviour completion deserves its own category and local category?

[scohen]

Suggestions:

• It would be exceedingly cool if we had different ideas of local. For example, functions defined in the current module are more local than functions that have been imported.
• Test cases should not be completed ever. Currently, they're coming in above snippets like defining new tests. I think they were completed before, but were ranked lower than the test snippets.