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-modules.scm

@@ -1,7 +1,8 @@
 ; Why `sex-modules`? Probably `modules` unit is reserved by chicken,
 ; things start to break.
 (declare (unit sex-modules)
-         (uses utils))
+         (uses sex-reader
+               utils))
 
 (import brev-separate
         (chicken file)
@@ -14,7 +15,7 @@
 
 (define +persistent-module-paths+ (list))
 
-(define (import-modules module-list)
+(define (get-public-forms module-list)
   ;; Module list is a list of symbols
   ;; How Sex handles modules:
   ;; For each module in a list, construct path, find module by path in
@@ -63,6 +64,7 @@
      (list)
      raw-forms)))
 
+;;; TODO: use semen facilities to analyze modules
 (define (process-public-interface-form form acc)
   (case (car form)
     ((pub)