Included cstddef in core.hpp (#146)
Included cstddef in core.hpp (#146)

With some library/compiler combinations, including a standard library
causes a clash with max_align_t, which is only present in cstddef. This
minor change helps avoid that situation.
KazDragon/terminalpp,KazDragon/terminalpp,KazDragon/terminalpp