Use source index to get completions from current package

NOTE: This could use some review as I'm not fully aware of the most 'idiomatic' ways of generating R objects / lists from C++ code.

This PR generates autocompletions from the source index when working on a package project.
Look at the file RSexp.hpp -- there are a family of "create" functions that that map from C++ types to R objects. I don't think we have one for a named list yet but we could easily add one.