z80: remove layer of indirection in most "next" calls

The only calls still using the stable ABI indirection are those
made before the next routine is created.

blk/243

@@ -5,7 +5,8 @@
 
 VARIABLE lblchkPS
 : chkPS, lblchkPS @ CALLnn, ; ( chkPS, B305 )
-: JPNEXT, 26 BJP, ; ( 26 == next )
+CREATE lblnext 0x1a , ( stable ABI until set in B300 )
+: JPNEXT, lblnext @ JPnn, ;
 : CODE ( same as CREATE, but with native word )
     (entry) 0 C, ( 0 == native ) ;
 : ;CODE JPNEXT, ;

blk/300

@@ -1,4 +1,4 @@
-PC ORG @ 0x1b + ! ( next )
+lblnext BSET PC ORG @ 0x1b + ! ( next )
 ( This routine is jumped to at the end of every word. In it,
   we jump to current IP, but we also take care of increasing
   it by 2 before jumping. )