Support path aliasing

[LPGhatguy]

If two partitions point to the same path or one of the paths contains the other, Rojo right now will do weird stuff like name your instances incorrectly or silently drop some changes.

This support needs to happen in a couple places:

• Update the IMFS and file watcher interface to skip paths its already seen when building the sync point index
• Change all of the RbxSession and snapshot code to change HashMap<PathBuf, T> to HashMap<PathBuf, Vec<T>> and adjust algorithms to make the split less painful.

[LPGhatguy]

A lot of the baseline refactoring work needed to do this has been done, now it should just be down to flipping a type to use Vec (or maybe SmallVec) and change a couple cases in RbxSession to be loops.

The other part that I've been working is in the in-memory filesystem code, which needs to take care not to watch or read paths redundantly. It's not strictly necessary to support path aliasing, but definitely helps clean it up.