For entries with fixed set of fields having an initializer list option comes
as nice syntactix sugar and spares the need of the explicit reserve() call,
and making sure to use the right size value there.
Due to fields having either a numeric value or a string one, this needs some
bit of C++ vodoo to avoid extra costs of having both a QString & a longlong
field per every entry, which would also mean a QString constructor &
destructor call also for each numeric field of the init list.
The UDSEntryBenchmark showed that those constructors & destructors add
notable cossts over explicit code to reserve & fill a UDSEntry, while using
a union with a QString mapped onto a char array gets numbers close.