[ADD] manage variables *

Also:
- dev:
  - add docu
  - add fixes in TODO
  - add FIXME

dev/TODO

@@ -6,7 +6,6 @@
 - signals
 	- ^C, ^D, ^\
 - variables
-	- normal variables
 	- environment variables
 	- $?
 - builtins:
@@ -17,3 +16,8 @@
 	- unset		(with no options)
 	- env		(with no options or arguments)
 	- exit		(with no options)
+- fix
+	- `$ |` throws 2 errors
+	- empty variables aren't freed
+
+(also search TODO and FIXME in the files)