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.

tests/run.scm

@@ -1,4 +1,5 @@
-(declare (uses sexc))
+(declare (uses fmt-c-writer
+               semen))
 
 (import
   (chicken process)
@@ -7,7 +8,7 @@
   test)
 
 (include "basic.scm")
-(include "types.scm")
+(include "semen.scm")
 
 ;;; Should be the last in the test suite
 (test-exit)