Implement tables and call_indirect (#12)

* Implement tables and call_indirect

* Restore comment about sig checking.

* Widen callee index on 64bit platforms.

filetests/call_indirect.wat

@@ -0,0 +1,50 @@
+(module
+  (type $indirect_sig (func (param i64) (result i64)))
+
+  (func $assert (param i32)
+    (block $ok
+      (br_if $ok
+        (get_local 0)
+      )
+      (unreachable)
+    )
+  )
+
+  (func $plus_1 (param i64) (result i64)
+    get_local 0
+    i64.const 1
+    i64.add
+  )
+  (func $minus_1 (param i64) (result i64)
+    get_local 0
+    i64.const 1
+    i64.sub
+  )
+
+  (func $main 
+    (call $call_indirect
+      (i32.const 0)
+      (i64.const 2)
+    )
+    (call $call_indirect
+      (i32.const 1)
+      (i64.const 0)
+    )
+  )
+
+  (func $call_indirect (param $func i32) (param $expected i64)
+    (call $assert
+      (i64.eq
+        (call_indirect (type $indirect_sig)
+          (i64.const 1)
+          (get_local $func)
+        )
+        (get_local $expected)
+      )
+    )
+  )
+  (start $main)
+  
+  (table 2 2 anyfunc)
+  (elem (i32.const 0) $plus_1 $minus_1)
+)