separate tags and typedefs

C has two namespaces and the database had one. A typedef of a tag's
name wiped it.

Tags, i.e. struct, enum and union, share one namespace; they move to a
table of their own. A name resolves the way C does: the ordinary
identifier first, the tag when that leads nowhere.

While resolving a typedef, the target was taken apart with cadr
whatever it was, so (typedef points (¤ point 4)) reported point's
fields as its own -- an array of four points claiming to be a point,
and a macro reaching through it with v->x. Only a name and
(struct|union|enum NAME) name a type now.

sex-macros.scm

@@ -26,8 +26,8 @@
            (import scheme
                    (scheme base)
                    (only sex-macros cat comment)
-                   (only types get-type-info get-fields get-underlying-type
-                         type-match map-fields))
+                   (only types get-type-info get-tag-info get-fields
+                         get-underlying-type type-match map-fields))
            ,@body)))
 
 (define (get-macro name)