* Define a generic allocator interface to enable static polymorphism for
allocators (see `GenericAllocator` in `src/allocators.zig`). Note that
`std.mem.Allocator` itself is considered a generic allocator.
* Add utilities to `bun.allocators` for working with generic allocators.
* Add a new namespace, `bun.memory`, with basic utilities for working
with memory and objects (`create`, `destroy`, `initDefault`, `deinit`).
* Add `bun.DefaultAllocator`, a zero-sized generic allocator type whose
`allocator` method simply returns `bun.default_allocator`.
* Implement the generic allocator interface in `AllocationScope` and
`MimallocArena`.
* Improve `bun.threading.GuardedValue` (now `bun.threading.Guarded`).
* Improve `bun.safety.AllocPtr` (now `bun.safety.CheckedAllocator`).
(For internal tracking: fixes STAB-1085, STAB-1086, STAB-1087,
STAB-1088, STAB-1089, STAB-1090, STAB-1091)
