generate JIT argument type information for methods
This enhances `CodegenRust.py` to output `JSTypedMethodJitInfo` structures where appropriate. This brings a notable speedup to tests like Dromaeo's `dom-attr/getAttribute`, which improves by several orders of magnitude with these patches applied.
If there are tricks for addressing the XXX comments, I would appreciate hearing them.
I think this addresses all of #6904.
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