This also allows `Instruction` to be `Copy`, which massively speeds
up `<Instructions as Clone>::clone` since it can now just `memcpy`
the bytes using SIMD instead of having to switch on every single
element. I haven't looked at the disassembly of `InstructionIter::next`
yet, it could be that there are even more improvements yet to be gained
from either:
* Only doing work on `BrTable` (this might already be the case depending
on the whims of the optimiser)
* Using `unsafe` to make it a noop (we really don't want to do this,
obviously, since it means that `Instructions` has to be immovable)
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