TODO: macros

[polgreen]

Extend parser to support more meaningful error messages for:
- recursive macros (we don't want to support this but we do want to check for cycles)
- macros in next statement
- primed assignments in macros

Additional things to support:
- We should definitely import macros when we import a module, otherwise import module becomes confusing (modules get imported as in test/test-module-import-0.ucl)
- we should maybe allow import macros via macro =module1. (as done for types in test/test-type-import.ucl), but this is less of a problem because it throws an error.
- nested macros (but not cyclic)