Add a 'call_indirect' builtin to emit arbitrary calls (might trap at runtime); Optimize 'for' loop compilation a bit

tests/compiler/std/hash.optimized.wat

@@ -26,7 +26,7 @@
      (i32.const 1)
     )
    )
-   (loop $loop|0
+   (loop $repeat|0
     (br_if $break|0
      (i32.ge_u
       (get_local $1)
@@ -53,7 +53,7 @@
       (i32.const 1)
      )
     )
-    (br $loop|0)
+    (br $repeat|0)
    )
   )
   (get_local $2)