Initial efforts at specifying inference in function bodies and initializers (#3803)

This is just the beginning of my effort; I'm sending it around to get
feedback on the style and general approach I'm taking.

What's included here:

- Some overview information, including some text about soundness
  guarantees (I want to include informal proofs of soundness as
  non-normative text along with the inference algorithm, so I need to
  define some terms).

- Details about coercions (not complete).

- Inference for the following expression types: `null`, numbers,
  boolean literals, string literals (including interpolation), symbol
  literals, `throw`, `this`, logical boolean expressions, and `await`.

Author: stereotype441