Update README with more details

Updates the README with a more detailed breakdown of the API, along with
a number of tweaks to the APIs including:

- Adds the `prop` keyword and class element type
- Adds the `init` keyword and class element type
- Updates field semantics to not transform to getter/setter pairs by default
- Adds the `access` property to the context object for private elements
- Refactors the metadata API to be more intuitive and well defined