Migrate from Scala macros to scalameta

[sullivan-]

There are some missing features in scalameta that make this impossible at the moment. But it seems like those features that we will need will come around relatively soon. I'm mostly looking at this Semantic API roadmap:

https://github.com/scalameta/scalameta/issues/604

The main thing I think we would need is: functionality to work with symbols: https://github.com/scalameta/scalameta/issues/609

Possibly stuff from this Term.tpe story as well: https://github.com/scalameta/scalameta/issues/611

There is an existing longevity branch, feat/meta, that has the last attempt at migrating to meta. When the next meta release comes out, we should dust off that branch, and see how much further we can get.