Reimplement wac-parser on wac-graph.

[peterhuene]

This PR is a complete reimplementation of wac-parser's resolution to be based on wac-graph.

It also contains an overhaul of the wac-graph API to facilitate its use from wac-parser:

• Reimplements CompositionGraph on top of petgraph::StableDiGraph, which eliminates needing to keep track of nodes internally to CompositionGraph.
• Packages now populate their type information into the graph's types collection rather than having their own collections.
• Splits the Error enum into discrete error types for each graph operation; this allows the AST resolver to properly attach span information to errors.
• Add type dependency edges for defined types, ensuring topological ordering.
• Fix encoding of defined type aliases so that each encoded alias refers to the export index of the previously encoded type.
• Added a debug formatter for CompositionGraph that outputs a DOT representation (used in resolution tests as well).
• Graph encoding now includes an optional producers custom section.
• Fixed toposort to be in ascending node index order for independent nodes.

The subtype checker implementation was reverted to the previous implementation, where invert and revert are called at specific points to change the type of check being performed; the newer implementation did not give the correct "expected, found" error messages for AST resolution.

wac resolve now outputs a DOT representation of the resolved composition graph instead of JSON.

A names custom section is now included in the output of an encoded WAC document.

Most of the deleted code comes from code that has already moved into wac-types and wac-graph.

Closes #79.
Fixes #78.
Fixes #76.
Closes #37.

[peterhuene]

When this and a fix for #81 lands, I plan on adding a publish script to CI and creating a 0.1.0 tag to publish to crates.io.

[rylev]

I'm seeing a stack overflow when running this against my own project. It seems that for some reason, a particular interface that we're merging into the types collection has an export (a resource) that depends on the interface itself - specifically the resource alias owner is the interface being merged. This causes an infinite loop since we've not yet registered this interface as being remapped.