we were moving as far as constructor arg, but afterwards we were copying to...

we were moving as far as constructor arg, but afterwards we were copying to member. this fixes that. https://stackoverflow.com/a/16725030/5257399

src/printf.h

@@ -21,7 +21,7 @@ public:
 class PrintableString : public virtual IPrintable
 {
 public:
-  PrintableString(std::string value) : value_(value) { }
+  PrintableString(std::string value) : value_(std::move(value)) { }
   uint64_t value();
 private:
   std::string value_;