This is an evolution of the old tools/defs/types.go file which
essentially had these data structs manually defined in a static go file.
Now, the parser should be able to construct this data structure on the
fly from input files and reference files.
This package will be the frontend for reading the JSONLD context
descriptions that specify ActivityStreams vocabularies. This will allow
ingesting publicly hosted or manually-created vocabularies and
generating an internal representation for later code generation.