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.

tests/types.module.scm

@@ -9,6 +9,7 @@
      map-fields
 
      get-type-info
+     get-tag-info
      get-fields
      get-underlying-type)
   "../types.scm")