common/file_util: Make IOFile's WriteString take a std::string_view

We don't need to force the usage of a std::string here, and can instead
use a std::string_view, which allows writing out other forms of strings
(e.g. C-style strings) without any unnecessary heap allocations.

src/common/file_util.h

@@ -257,8 +257,8 @@ public:
         return WriteArray(&object, 1);
     }
 
-    std::size_t WriteString(const std::string& str) {
+    std::size_t WriteString(std::string_view str) {
-        return WriteArray(str.c_str(), str.length());
+        return WriteArray(str.data(), str.length());
     }
 
     bool IsOpen() const {