Warn about sequences being passed to repeated parameters without :_*

In what case can I get an `Apply(fun)` where `fun` is not a `SymTree`?

I started with `fun.tpe.paramTypes` but when I wanted to limit false-positives, I needed to know if the *declaration* is using a type parameter, so I don't want the `asSeenFrom` substitution. I'll try it again, but I remember `fun.tpe` was the fully instantiated type, so didn't know if it came from a type variable or not.