added get_addr and jmp

tests/jmp_test.sl

@@ -0,0 +1,18 @@
+import stdlib.sl
+
+word main
+    5          # loop count
+    get_addr   # push loop target address (next instruction)
+    "gg" puts
+
+    swap       # bring count to top
+    1 -        # decrement
+    dup 0 > if
+        swap   # put addr on top
+        dup    # keep a copy for next iteration
+        jmp
+    else
+        drop   # drop count
+        drop   # drop addr
+    end
+end