editor: simplify I with the use of MOVE-

Also, delay the shadowing of DO..LOOP's I for as long as possible.

blk/102

@@ -12,5 +12,5 @@
         ENDCASE
     AGAIN
 ;
-
-
+( I masks DO..LOOP's I. Do it as late as possible. )
+: I _I ;

blk/108

@@ -1,16 +1,15 @@
 : _ilen ( length of str in IBUF )
     IBUF BEGIN C@+ EOL? UNTIL IBUF - 1- ;
-: I
-    IBUF _type EDPOS @ 64 /MOD ( cno lno )
-    1+ 64 * _cpos ( cno next-line-ptr )
-    SWAP 63 -^ _ilen ( nlp nb-of-chars-to-move ilen )
+: _I
+    IBUF _type EDPOS @ 64 MOD ( cno )
+    63 -^ _ilen ( rbuffsize ilen )
     2DUP > IF
-        SWAP OVER - 1+ ( nlp ilen nbc ) 0 DO ( a ilen )
-            SWAP 1- 2DUP -^ ( ilen a-1 a-ilen-1 ) C@ OVER C!
-            SWAP ( a ilen )
-        LOOP
-    ELSE DROP ( ilen becomes nbc )
+        SWAP OVER - ( ilen chars-to-move )
+        SWAP EDPOS @ _cpos 2DUP + ( ctm ilen a a+ilen )
+        3 PICK MOVE- ( ctm ilen )
+        SWAP DROP ( ilen )
+    ELSE DROP ( ilen becomes rbuffsize )
     THEN
-    SWAP DROP DUP IBUF EDPOS @ _cpos ROT MOVE ( ilen )
+    DUP IBUF EDPOS @ _cpos ROT MOVE ( ilen )
     EDPOS +! EDPOS @ 64 / _pln
 ;