Refactor string conversion between RCT2 and UTF8

Use Win32 API for conversion on Windows for non-1252 code pages instead of built-in tables.

src/openrct2/core/IStream.cpp

@@ -37,17 +37,13 @@ utf8 * IStream::ReadString()
 
 std::string IStream::ReadStdString()
 {
-    std::vector<utf8> result;
-
+    std::string result;
     uint8 ch;
     while ((ch = ReadValue<uint8>()) != 0)
     {
         result.push_back(ch);
     }
-    result.push_back(0);
-
-    std::string resultString(result.data(), result.data() + result.size());
-    return resultString;
+    return result;
 }
 
 void IStream::WriteString(const utf8 * str)