I'm not sure if `CodegenRust.py` was supposed to remove the underscore,
but we end up exposing `type_` to javascript which is obviously wrong.
There's no need to rename the method in the first place, because `Type`
(with a capital T) is not a rust keyword.
Testing: Covered by existing web platform tests
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>