Fix realloc for non-trivial types.

Fix realloc for non-trivial types.

If a non-trivial type RequiresInitialization, then we unfortunately can't simply rely on realloc to move the memory to a new buffer. If the data type contains self-referencing pointers (as does AnnoyingScalar in tests), then those pointers become invalid. Instead, we need to allocate a new buffer and copy-construct (or move-construct) existing elements.

This fixes some failing tests in sparse_block, which tickled the bug.