Add missing/new imports and requires

[brandonwillard]

Per this discussion, for some of the new Hy update rules, we will need to add new import and require statements.

The additions will need to be scoped somewhat intelligently (e.g. the outer-most scope that encompasses more than one use of the import/require). For standard Python-scoped objects, this isn't too complicated, but for macros—especially pre hylang/hy#1682—the naive per-line/in-place addition might be necessary.