modularize sex

Also rename macros to sex-macros, module-system to sex-modules for
clarity, uniformity, and to avoid name clashes with Chicken's
units/modules named "macros" and "modules"

main.scm

@@ -1,6 +1,5 @@
 ;;; The purpose of this file is to compile it to the only
 ;;; .o that has main entry point.
 
-(declare (uses sexc))
-
+(import sexc)
 (main)