GREEDY_REALLOC_APPEND: Make more type safe

GREEDY_REALLOC_APPEND: Make more type safe

Previously, GREEDY_REALLOC_APPEND would compile perfectly fine and cause
subtle memory corruption if the caller messes up the type they're passing
in (i.e. by forgetting to pass-by-reference when appending a Type* to an
array of Type*). Now this will lead to compilation failure