use an llvm::SmallDenseMap instead of llvm::DenseMap for ASTInterpreter's SymMap

src/codegen/ast_interpreter.cpp

@@ -77,7 +77,7 @@ public:
 
 class ASTInterpreter : public Box {
 public:
-    typedef ContiguousMap<InternedString, Box*> SymMap;
+    typedef ContiguousMap<InternedString, Box*, llvm::SmallDenseMap<InternedString, int, 16>> SymMap;
 
     ASTInterpreter(CompiledFunction* compiled_function);