don't prematurely unpack structs with complex types; support for convert patterns that use `default`

- unique0 and priority
- uniqueness on if statements
- preliminary discard-only parsing of assertions
- parameters with alias typenames

allow for stray semicolons in module items; allow for multiple struct field declarations on one line

This should make it much easier to add support for ``, `", macros with
arguments, etc., in the future.