Json cmake follow

This has been around for several years. Let's finally merge it in. The main advantage is that json-based generators implement a lot of checks that can be turned on and off and which guarantee consistency of the tree. Html files are a bit nicer than before. Generation of C files is significantly faster, which speeds-up the overall compilation.

Also, by adopting JSON as a concept, we can use it in other parts of the compiler to describe things and to serialize/deserialize things.