Only changes to the ASTs required fixing:
* Asttypes.label -> Asttypes.arg_label
* New definition of Parsetree.constant, shadowing Asttypes.constant
* Addition of Typedtree.rec_flag
* Removal of Typedtree.optional

Required changes:
* Ignore ocaml.doc and ocaml.text attributes, which are now
  automatically extracted from comments.
* Swap type definition attribute syntax in test/* files.

Other changes:
* Add opam package file and `make init` command for dependencies.
* Change "Automatically Generated Attributes" documentation to comments.