For enum values that are constructed, not literal integers, we need to
parse the inner / implicit expression. For example:
```
GIT_DIFF_REVERSE = (1u << 0)
```
The API documentation generation is useful (in `--strict` mode) to
determine whether all our APIs have sufficient documentation. Add a
`--validate-only` mode that will run the validation without emitting the
API JSON. This is useful for ensuring that the documentation is
complete.
libgit2 has a new documentation generator that generates API schema from
our headers, then produces reference documentation that is included into
the website directly.
