Add cancel function.

This should allow better stack unwinding on a fiber that
no longer needs to complete.

src/include/janet.h

@@ -1131,6 +1131,7 @@ enum JanetOpCode {
     JOP_NEXT,
     JOP_NOT_EQUALS,
     JOP_NOT_EQUALS_IMMEDIATE,
+    JOP_CANCEL,
     JOP_INSTRUCTION_COUNT
 };