accept pub enum, and enums used as types

Implement pub support for enums, and also anonymous enums declared
in-place.

Also give the cdr of a `pub' form the form's own location, so a
error about what follows `pub' can say where it was written

tests/modules/greet.sex

@@ -11,6 +11,8 @@
 
 (pub struct greeting ((text (* const char)) (times int)))
 
+(pub enum mood (cheerful grumpy))
+
 ;;; 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.