7260: Render Fn* trait objects and impl types as rust does r=matklad a=Veykril
Also fixes raw ptr impl types being rendered ambiguously with multiple predicates.
This moves out the `FamousDefs::Fixture` into its own file as well, cause I figured it is big enough to get its own file at this point + we also get highlighting this way when editing it.
Fixes#3012
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
39167b97d8