[wasm] Refactor encoder.h to use a proper buffer and remove OldFunctions section.

test/unittests/wasm/ast-decoder-unittest.cc

Index: test/unittests/wasm/ast-decoder-unittest.cc
diff --git a/test/unittests/wasm/ast-decoder-unittest.cc b/test/unittests/wasm/ast-decoder-unittest.cc
index f9b78f14da5aa668cd7646448535c8dc5d4afcfb..55c7095f80f506e31333a721d1ee7d6a5a4edd8d 100644
--- a/test/unittests/wasm/ast-decoder-unittest.cc
+++ b/test/unittests/wasm/ast-decoder-unittest.cc
@@ -1169,14 +1169,13 @@ class TestModuleEnv : public ModuleEnv {
     return static_cast<byte>(mod.signatures.size() - 1);
   }
   byte AddFunction(FunctionSig* sig) {
-    mod.functions.push_back({sig,      // sig
-                             0,        // func_index
-                             0,        // sig_index
-                             0,        // name_offset
-                             0,        // name_length
-                             0,        // code_start_offset
-                             0,        // code_end_offset
-                             false});  // exported
+    mod.functions.push_back({sig,  // sig
+                             0,    // func_index
+                             0,    // sig_index
+                             0,    // name_offset
+                             0,    // name_length
+                             0,    // code_start_offset
+                             0});  // code_end_offset
     CHECK(mod.functions.size() <= 127);
     return static_cast<byte>(mod.functions.size() - 1);
   }