Implement new patch infrastructure.

[olafurpg]

This commit is too big, I'm sorry. Main changes:

- Patch(from, to, replace) replaced with TokenPatch(token, newToken)
  with helpful combinators such as Add{Left,Right}
- New high-level TreePatch datatype with can be converted to TokenPatch.
- Implemented two tree patches, AddGlobalImport and RemoveGlobalImport.
- Implemented OrganizeImport, which is a prerequisite to use import tree
  patches.
- OrganizeImports
  - orders imports by configurable groups
  - removes unused imports using scalac -Ywarn-unused-import
    infrastructure. The implementation is requires hijacking a few
    private fields in g.analyzer using evil reflection hackery.
  - option to expand relative imports
  - handles renames
  - configurable "always used" to force keeping an import such as
    acyclic.file

Known bugs:

- introduces false import on non-existing cats.data.UUID in circe
- makes an import unused by expanding relative imports to
  fully-qualified imports, breaking compilation in projects
  with -Ywarn-unused-import (like cats) unless you run scalafix twice
- crashes on scala.meta parser bug (in akka), not scalafix problem
  really.

[olafurpg]

I'm struggling to locally reproduce the cats it test. Will need to leave it for tomorrow.

[olafurpg]

Seems ci is broken for some reason, opening another PR

[ShaneDelmore]

This whole file is great, I love how readable you have made it. I did one read through and immediately understood the code and how to use it 👍