Support TASTy version 20.0 [Dotty 0.23]

https://github.com/lampepfl/dotty/pull/8170

https://github.com/lampepfl/dotty/pull/8212

- be aware of an optional third type tree in a type bounds type tree, which is the alias of an opaque type

https://github.com/lampepfl/dotty/pull/8428