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

`fun` is not guaranteed to be a `SymTree`, so this could NPE. See https://github.com/sbt/sbt/commit/3e9a7c7a31443b4e4aa4950cac78469da774b606 for an example of this in SBT's macros.

Furthermore, `tree -> symbol -> type` is lossy; you discard the subsitutions made by as seen from. I think `fun.tpe.paramTypes` is what you need instead.

This might not be a problem as things are written, but would be a problem if you tried @adriaanm's alternative to approximate the param type.