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/Makefile

@@ -14,7 +14,7 @@
 
 SEXC ?= ../../sexc
 
-EXPECTED = hello, world\nhello, sex\n2 greetings\ntext times \nmood 1
+EXPECTED = hello, world\nhello, sex\n2 greetings\ntext times \ntext times \nmood 1
 
 check:
 	@$(SEXC) greet.sex -c -o greet.o

tests/modules/greet-app.sex

@@ -21,6 +21,9 @@
   (printf "%d greetings\n" greet-count)
   (describe-fields greeting)
   (printf "\n")
+  ;; ...and through the imported typedef for it
+  (describe-fields greeting-t)
+  (printf "\n")
   (var m (enum mood) grumpy)
   (printf "mood %d\n" m)
   (return 0))

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.