JSON Schemata for AST (and potentially all json involved in the compilation process)

Eventually, all json occurring as input or output of the compiler should be organized into JSON schemata.
The first most important candidate would be the AST.
(brought up by @veniger)

The issue names the compiler's AST and other JSON input and output, but no files, tests, or entry points. Start by locating how the compiler currently serializes and consumes AST JSON, then define the scope of the first schema and how it will be validated. Done means the AST JSON is organized under a documented schema, with the remaining compiler JSON scope clearly identified.

Written by the indexing model from the issue text.