process imported forms as toplevel, not as text

1. register imported types
2. protect against multiple imports (diamond, circular)

tests/modules/greet.sex

@@ -9,6 +9,16 @@
 
 (pub var greet-count int 0)
 
+(pub struct greeting ((text (* const char)) (times int)))
+
+;;; 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.
+(pub defmacro (describe-fields type)
+  `(do ,@(map-fields type
+           (lambda (name field-type)
+             `(printf "%s " ,(symbol->string name))))))
+
 (pub fn greet ((name (* const char))) void
   (++ greet-count)
   (printf "hello, %s\n" name))