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

tests/compiler/scoped.optimized.wat

@@ -10,7 +10,7 @@
  (func $start (; 1 ;) (type $v)
   (local $0 i32)
   (block $break|0
-   (loop $loop|0
+   (loop $repeat|0
     (br_if $break|0
      (i32.ge_s
       (get_local $0)
@@ -23,14 +23,14 @@
       (i32.const 1)
      )
     )
-    (br $loop|0)
+    (br $repeat|0)
    )
   )
   (block $break|1
    (set_local $0
     (i32.const 0)
    )
-   (loop $loop|1
+   (loop $repeat|1
     (br_if $break|1
      (i32.ge_s
       (get_local $0)
@@ -43,7 +43,7 @@
       (i32.const 1)
      )
     )
-    (br $loop|1)
+    (br $repeat|1)
    )
   )
   (call $scoped/fn