fix: mark ByteArray.Iterator Inhabited instance as noncomputable

The Iterator structure has a custom SizeOf instance that depends on runtime
array size, making the Inhabited instance noncomputable. This change explicitly
marks it as such and adds a comment explaining why.

Related: https://leanprover.zulipchat.com/#narrow/channel/113489-new-members/topic/Compiler.20error.20with.20sizeOf/with/561532734

Co-authored-by: nomeata <148037+nomeata@users.noreply.github.com>