resolve typedefs in get-fields and map-fields

Now these macro helpers take in account the possibility of typedefing
one type to another, and correctly find the one intended

tests/modules/greet.sex

@@ -13,6 +13,8 @@
 
 (pub enum mood (cheerful grumpy))
 
+(pub typedef greeting-t (struct greeting))
+
 ;;; Compile-time reflection across the module boundary: both the macro
 ;;; and the struct it asks about are exported, and the importing unit
 ;;; has to know the struct's fields to expand this.