Add feature: navigating the current file's syntax tree

You know those boxes that Visual Studio and others have that let the
user jump to different types and methods etc? This is that :)

Long explanation:

I wrote a feature that returns the locations of different nodes in the
current file's syntax tree. These are implemented as QuickFixes, and
they make it possible to write plugins that let the user navigate to:

- different types in the current file
- members in the current type

I also added one option that returns the current AST as a tree
structure. This makes it possible to write an interface that mimics
the tree-like representation of the AST.