Experiment with async handling missing imports; minor fixes

We don't actually do anything yet, but the stuff in place seems
to work. That said, "init" actually runs very fast, so it may
be feasible to just do it synchronously. I guess we'll have to see
how slow searching for the right import will be.... We may want
to consider separating this to a separate bufWrite endpoint, so
init can pre-cache/index stuff, and bufWrite can do the import
searching.
Split import checking stuff out of init and into "update"
Experimental import suggestions

Refs #1

Still no file/UI changes, but the search runs!
Experiment with insert auto-imports (position is lame)

Refs #1
Move type caching to SourceCL; add tests for type caching
Improvements to auto-import UI, including better grouping

Refs #1

Also includes a test for the import insertion. We don't yet
handle new package TLDs, though
Adjust cursor after inserting imports for seamlessness

Refs #1
"fixit" refactoring interface to resolve ambigous imports

Closes #1