Minor bug in codegen_base.cpp

[benjaminfaul2]

Found a minor typo in codegen_base.cpp at line 141:

   virtual void visit(const int64_field_instruction *, void *) override {
-    name_ = "mfast::uint64";
+    name_ = "mfast::int64";
   }

   virtual void visit(const uint64_field_instruction *, void *) override {
-    name_ = "mfast::int64";
+    name_ = "mfast::uint64";
   }