Move EMPTY to core words

It's a quite useful word and it doesn't make sense to reload it from
disk after each usage.

blk/152

@@ -1,6 +1,5 @@
 '? CASE NOT [IF] 1 2 LOADR+ [THEN] DROP ( from '? )
 '? FILL NOT [IF] 3 LOAD+ [THEN] DROP
-'? EMPTY NOT [IF] 4 LOAD+ [THEN] DROP
 '? WIPE NOT [IF] 5 LOAD+ [THEN] DROP
 
 

blk/156

@@ -1,16 +0,0 @@
-: EMPTY
-    LIT< _sys FIND NOT IF ABORT THEN
-    DUP HERE ! CURRENT ! ;
-
-
-
-
-
-
-
-
-
-
-
-
-

blk/372

@@ -11,6 +11,4 @@
     WORD( HERE !  ( w )
     PREV CURRENT !
 ;
-
-
-
+: EMPTY LIT< _sys FIND IF DUP HERE ! CURRENT ! THEN ;