Completions are unavailable until file is saved

[andyw8]

Currently we only index when a file is saved, meaning that completions will be unavailable for newly-added symbols within the same file. This could be unexpected, especially if people are coming from other editor setups which behave differently.

A couple of options that come to mind:

• Index the current file on changes
• Index the current file on-the-fly when the completion is triggered

We should also consider the case of multiple unsaved files, with one referencing the other.