WIP: Support nested partitions and partitions directly targeting services

[LPGhatguy]

Closes #112. Closes #102. Closes #105.

This makes the project format significantly easier to reason about by turning both SourceProjectNode and ProjectNode into structs instead of enums with associated data, like they are today.

Error messages should be much better, and it opens up the possibility to specify $className, $children, and $properties on the same partitions that you can specify $path on. It should be a backwards-compatible change, except in the case of previously malformed projects.

TODO

• ~Custom Deserialize impl to give better error messages on project structure (guide)
• Documentation on correct project structure Docs need more visiting than just this
• More exhaustive tests
• Fix snapshot system blasting away children

Major Changes

• Removed PathMap's entry API because it wasn't sound
• Collapsed SourceProjectNode and ProjectNode from enums into structs with optional fields
• Replaced 'metadata per path' concept with just a set of IDs that a path update affects
• Removed the concept of snapshot context, which makes snapshotting pure now (!!)
• Made snapshots correctly encode the project nodes they came from in order to correctly restore children and properties defined in-project
• Implemented a snapshot testing system that commits expected outputs into source control and compares them at test time. This required creating a partial ordering for some objects to make output more deterministic.

[coveralls]

Coverage remained the same at 80.952% when pulling 0cc79ce on nested-partitions-and-stuff into 38e3c19 on master.

[LPGhatguy]

It looks like we might have to start storing actual ProjectNode objects in the path-per-instance metadata object so that we can correctly reconstruct children and properties specified by the project on each change.

[LPGhatguy]

Build looks like it might be failing due to how pretty assertions does its comparison?