refactor(bib)!: make InputReference an enum

[bdarcus]

Split input model into structural classes, much like the very first CSL implementation.

Closes: #93

---

TODO:

1. [X] add identifiers
2. [X] add a NumOrStr type for use in locators, etc.

This is a pretty major internal change.

Like the very first CSL implementation, almost 20 years ago, this splits InputReference into different classes. A Book type, for example, is a Monograph, while a JournalArticle is a SerialComponent.

type: article
title: Article title
  parent:
    type: academic-journal
    title: Journal title

Aside: I want to allow parents to be referenced by citekey, but I haven't figured out how to do that technically. My suspicion is it will take another major internal change; changing bibliography to be an IndexMap and writing some custom deserialization code. So it's not worth the hassle ATM.

This also addresses some other internal issues I was unhappy with.

For example, the render traits now return Option<String>, and surrounding code is updated to better handle missing data.

[bdarcus]

Thoughts on this @denismaier?

I'm not so worried about the details of the types, so much as getting the basic model right, since it touches a lot of the internal code, and is also pretty outward-looking.

This introduces a basic "hierarchical model" talked about for Zotero from the beginning, but never implemented.

From the chicago examples file:

hutt:
  type: chapter
  issued: "2011"
  author: 
    family: Hutter
    given: Michael
  title: 
    main: Infinite Surprises
    sub: Value in the Creative Industries
  parent:
    type: book
    issued: "2011" # currerntly required in both places
    title: 
      main: The Worth of Goods
      sub: Valuation and Pricing in the Economy
    editor: 
      - family: Beckert
        given: Jens
      - family: Aspers
        given: Patrick
    publisher:
      location: New York
      name: Oxford University Press
  pages: 201-220

The goal is ultimately to be able to do this, though I don't know how ATM:

title: chapter
parent: doe21

I also added the start of identifiers.

[bdarcus]

I had to close this because of a mistake. See #95.

[denismaier]

I think this looks reasonable.

With this maybe even a relatedItem feature might become feasible?

[bdarcus]

Yes.