Refactor Tokenizable storage

[plv]

Currently, Inodes store Tokenizable:

• Directory stores them so that future children can inherit this tokenizer.
• Document stores them because Document is also Tokenizable.

However, this can be roundabout. As my comment in Inode.kt explains:

// TODO:  Storing a Tokenizable in each Directory and Document is a bad idea. It wastes space with no real time benefits
// (esp. if API users may supply a tokenizer that is not a singleton). Also, this can leak if we accidentally keep
// references to a Tokenizable that is also a Document, even after we remove the Document from the DocumentStore.
// I'm crunched for time as of writing this comment, but the smarter thing to do would be to dynamic dispatch
// using a map of some sort.