This makes use of Petgraph for representing the dependency graph and uses a
separate data structure for both string interning and indexing by symbol
name.
This is garbage code. Do not use it. It is intentionally throwaway.
While I've researched Rust, I haven't actually _used_ it for a project, so
this is a combination of me exploring various ways of accomplishing the
problem and forcing myself to learn certain aspects of the language.
I'll likely be using petgraph, and this also currently lacks symbol
abstractions. This commit also performs far too much heap allocation
copying strings around. But it _does_ perform the topological sort.
Since this only stores the symbol name, it lacks enough information about
the symbol to perform a proper linking.
Mike Gerwitz