Replace the raw array in ReservedVector by std::array

Summary

In ReservedVector the data was stored as raw array. This has 2 problems: first, we cannot allocate a raw array of size 0. This may happen in generic code, second, the raw array with large size may result in a compiler warning

note: variable tracking size limit exceeded with -fvar-tracking-assignments, retrying without

I'm not exactly sure why, but the change to std::array solved this warning for me.