go.mod; fix unit tests
- Updated dependencies and switch from gopkg to go mod.
- Palette was moved from stack to internal; it was never meant to be exposed publicly.
- Please ping the maintainer if this breaks you.
- Fixed unit test to work on go1.11 and later.
- Stopped using github.com/maruel/ut.
More refactoring in package stack is planned but this will be tagged as v2 since this will change the signature.