After this commit, every non-key field is treated as if it was optional
while every key field (plus every auto-generated `Kind` field) - as if
it was required.
This allows to avoid having to specify the `rootType` (stable within
the entire tree) manually each time a reference is introduced.
Thanks to that, a much nicer syntax of:
```yaml
- name: MyTypeField
reference-to: TypeDefinition
```
can replace the current one:
```yaml
- name: MyTypeField
reference:
pointeeType: TypeDefinition
rootType: Binary
```
Here we start restructuring the the schemas YAML such that it
contains a top-level `version` field. In this commit we only modify
the YAML, the build system changes will come later, to avoid
enormous commits.
These schema files are currently generated at build time by
collecting various TUPLE-TREE-YAML comments using the
`extract_yaml.py` script. In this commit we move them to the codebase
but we don't use them yet, this will be done at a later commit for
the sake of readability.
Model classes are now described by a YAML document, which is used to
generate C++ headers containing classes and all the boilerplate
required for YAML serialization/deserialization, usage in
SortedVectors, etc. See the README in include/revng/Model for more
info.
Ivan Krysak