split semantic processing and fmt-c code generation

Introducing Sex SEMantic ENgine: the semen.
Also split reader to other file (it can be replaced in the future).
Macro expansion inside Sex code doesn't work yet, and it must be done
in semen, not during fmt-c generation as before.

sex-macros.scm

@@ -19,11 +19,17 @@
 (define (get-macro name)
   (eval (get name 'sex-macro)))
 
-(define (macro? form)
+(define (sex-macro? form)
   (and (list? form)
        (symbol? (car form))
        (get (car form) 'sex-macro)))
 
+(define (apply-macro form)
+  (assert (sex-macro? form)
+          (fmt #f (car form) " is not a macro"))
+  (apply (get-macro (car form))
+         (cdr form)))
+
 (define (defmacro form)
   (let ((arglist (car form))
         (body (cdr form)))