864: Fix handling of generics in tuple variants and refactor a bit r=matklad a=flodiebold
(The problem was that we created separate substitutions for the return value, so we lost the connection between the type arguments in the constructor call and the type arguments of the result.)
Also make them display a tiny bit nicer.
Fixes#860.
Co-authored-by: Florian Diebold <flodiebold@gmail.com>
Co-authored-by: Florian Diebold <florian.diebold@freiheit.com>
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