sema: add `prev` to ValueArena to allow freeing previous arenas when new ones are created during re-analysis

In semaDecl, it was possible for a new ArenaAllocators state to replace an existing one that
hadn't been freed yet. Instead of the ref_count (which was made redundant by adding
the allocator parameter to `release`), I now store a pointer to the previous arena, if one exists.

This allows a recursive deinit to happen when the last arena created is destroyed.