Adapt vulpix to SAM types

[felixmulder]

When moving to >= 2.12 after #2827 is merged, we can make vulpix use SAM types instead of doing things like:

new Runnable { ... } we can do () => { ... }.

[3shv]

I can work on this. So, only the files under the package dotty.tools.vulpix should be affected right?

[felixmulder]

That is correct, but if you want, you can also adapt the things in the dotty.tools.dotc.interactive package IIRC :)

[3shv]

Raised a PR here
I found only two places where () => {...} it can be used. Please have a look and let me know if its fine.