grammar.peg didn't get updated along with grammar.peg.go in arv's last patch,
so that needed to be fixed. Also, pkg.Parsed had its own field named Types, which
shadowed the one it got by embedded types.Package. This only came into play when
generating code for packages pulled out of a dataset. Since arv had to manually
patch up all generated code in his last patch, he never hit this issue and I
missed it in review.
Now, go generate passes once more. Yay