Make Object pointer-sized

[SuchAFuriousDeath]

This is a working preview of the long-awaited (and promised) Object refactor.

The design is 90% me + 10% advice from Codex to fill in the gaps that I couldn't quite get right. Most of the actual code changes were done by Claude Code, because they're mostly mechanical, I iterated with him until I was reasonably happy with everything.
I'm not fully satisfied with the design and I don't expect this to get merged without any adjustments but I have to post it here to get more eyes on it, or it won't get finished at all.

Imperfections:

1. define_variants and the definition of the Object through the tagged_trait_object, that is one thing I can't quite figure out. Imo the tagged_trait_object should macro should not exist at all.
2. the way it is now, it's not reusable for stuff like DisplayObject (and similar enums). I also can't quite figure out how to do this in a satisfying way (and without making it complicated and confusing).
3. Probably some other stuff too but I don't remember anymore :D

As a sidenote, with the traits that currently exist in this PR, I think that handles (Stuff like ObjectHandle, NetStreamHandle, ScriptObjectHandle, MovieClipHandle and FileReferenceObjectHandle) could be generalized rather than having one type for each stashable Object type. So that's potentially something nice for the future.

Also, review commit-by-commit to preserve sanity.

[Lord-McSweeney]

This worsens a getslot microbenchmark of mine by ~10%. Here's the codegen for op_get_slot with this PR vs on current master:

Master:

  sub    $0x18,%rsp
  mov    %rdi,%rax
  mov    0x18(%rdi),%rcx
  mov    0x20(%rdi),%rdi
  dec    %rdi
  cmp    %rcx,%rdi
  jae    a8
  mov    0x10(%rax),%rcx
  shl    $0x4,%rdi
  movups (%rcx,%rdi,1),%xmm0
  movaps %xmm0,(%rsp)
  mov    (%rsp),%rdx
  lea    -0x2d(%rdx),%r8
  cmp    $0x1,%r8
  jbe    63
  cmp    $0x2c,%rdx
  ja     76
  mov    0x8(%rsp),%rax
  cmp    0x18(%rax),%rsi
  jae    8f
  add    %rdi,%rcx
  mov    0x10(%rax),%rax
  shl    $0x4,%rsi
  movups (%rax,%rsi,1),%xmm0
  movups %xmm0,(%rcx)
  xor    %eax,%eax
  add    $0x18,%rsp
  retq
(error/panic cases)

With this PR:

  mov    %rdi,%rax
  mov    0x18(%rdi),%rcx
  mov    0x20(%rdi),%rdi
  dec    %rdi
  cmp    %rcx,%rdi
  jae    a7
  mov    0x10(%rax),%rcx
  shl    $0x4,%rdi
  movups (%rcx,%rdi,1),%xmm0
  movaps %xmm0,(%rsp)
  movzbl (%rsp),%edx
  cmp    $0x1,%dl
  jbe    62
  cmp    $0x6,%dl
  jne    75
  mov    0x8(%rsp),%rax
  test   %rax,%rax
  je     75
  cmp    0x58(%rax),%rsi
  jae    8e
  add    %rdi,%rcx
  mov    0x50(%rax),%rax
  shl    $0x4,%rsi
  movups (%rax,%rsi,1),%xmm0
  movups %xmm0,(%rcx)
  xor    %eax,%eax
  add    $0x18,%rsp
  retq
(error/panic cases)