Rollup merge of #106739 - WaffleLapkin:astconv, r=estebank

Remove `<dyn AstConv<'tcx>>::fun(c, ...)` calls in favour of `c.astconv().fun(...)`

This removes the need for <>><><><<>> dances and makes the code a bit nicer.

Not sure if `astconv` is the best name though, maybe someone has a better idea?