OP_SELF restricted to constant short strings

Optimize this opcode for the common case. For long names or method
calls after too many constants, operation can be coded as a move
followed by 'gettable'.

lopcodes.h

@@ -256,7 +256,7 @@ OP_SETFIELD,/*	A B C	R[A][K[B]:shortstring] := RK(C)			*/
 
 OP_NEWTABLE,/*	A B C k	R[A] := {}					*/
 
-OP_SELF,/*	A B C	R[A+1] := R[B]; R[A] := R[B][RK(C):string]	*/
+OP_SELF,/*	A B C	R[A+1] := R[B]; R[A] := R[B][K[C]:shortstring]	*/
 
 OP_ADDI,/*	A B sC	R[A] := R[B] + sC				*/