process imported forms as toplevel, not as text

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

tests/modules/Makefile

@@ -7,10 +7,14 @@
 # module's object survives being passed after `--'. Get any of them
 # wrong and this fails to link -- or, in the `pub var' case, links and
 # quietly counts into a private copy.
+#
+# It also checks what only a second translation unit can check: that an
+# imported type reaches the type database, by expanding a macro that
+# reads the imported struct's fields.
 
 SEXC ?= ../../sexc
 
-EXPECTED = hello, world\nhello, sex\n2 greetings
+EXPECTED = hello, world\nhello, sex\n2 greetings\ntext times 
 
 check:
 	@$(SEXC) greet.sex -c -o greet.o

tests/modules/greet-app.sex

@@ -7,6 +7,9 @@
 ;;; as a prototype and `greet-count' as an extern. Both keep external
 ;;; linkage, so they refer to the one definition in greet.o rather than
 ;;; to private copies.
+;;;
+;;; Also check that import populates type-database, by means of
+;;; describe-fields macro, which should work on imported type.
 
 (include stdio.h)
 
@@ -16,4 +19,6 @@
   (greet "world")
   (greet "sex")
   (printf "%d greetings\n" greet-count)
+  (describe-fields greeting)
+  (printf "\n")
   (return 0))

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))