Fix 2 UBs, and cleanup GcRefCell

[hansl]

• Empty enums are uninhabited types and referencing them is UB.
• Removed the ref in GcRefMut<> and GcRef<> in favor of NonNull<> for good hygiene.
• Simplified the type of GcRefMut by having a ref to the flags instead of keeping a ref to the GcRefCell itself.
• Unconstrained the types for Deref and DerefMut for Ref/RefMut.
• Little things, like using assert_ne instead of assert!(... != ...), or typos in comments, etc.

[codecov]

Codecov Report

❌ Patch coverage is 61.11111% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.27%. Comparing base (6ddc2b4) to head (89e7fc7).
⚠️ Report is 530 commits behind head on main.

Files with missing lines	Patch %	Lines
core/gc/src/cell.rs	52.72%	26 Missing ⚠️
core/engine/src/interop/into_js_arguments.rs	0.00%	1 Missing ⚠️
core/engine/src/object/builtins/jsarraybuffer.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4422      +/-   ##
==========================================
+ Coverage   47.24%   51.27%   +4.02%     
==========================================
  Files         476      502      +26     
  Lines       46892    51291    +4399     
==========================================
+ Hits        22154    26297    +4143     
- Misses      24738    24994     +256     

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[nekevss]

Looks great! Had a couple nits and a question to start

[jedel1043]

LGTM!