Port WMMatching.scala to scala 2.13

[ornicar]

This is part of the lila rewrite to scala 2.13.

We need to port WMMatching.scala, because Traversable is gone in 2.13 collections. Please help. The file is standalone and can be compiled outside of lila.

Here's a minimal scala/sbt project with this file only: https://github.com/ornicar/wmmatching