Produce JSON schema for stable MIR JSON (from a corpus)

[jberthold]

We need a JSON schema for Stable MIR JSON.
A possible way to start creating the schema would be to run all the test-ui tests and retain the output, then analyse the output JSON to create a first version of the schema.
This first version needs to then be double-checked against the Rust code that produces the data, and enhanced with comments and descriptions about what the fields mean.