Dotty port

[jodersky]

• changes the build configuration to cross-build cask and all example apps against Scala 2.13 and Dotty (currently 0.27.0-RC1)
• reimplmenents the macros from the 2.13 version in the Dotty version

Note: ~this PR is still WIP~, as some items remain to be done:

• [x] (easy) check that annotated decorators are of coherent input-output types. In other words, reimplement seqify in the dotty version
• [x] (needs more thought / currently blocked) fix the implicit ambiguity error when returning json responses (see this discussion on Gitter for more background)

Other items that can be tackled after this is merged:

• implement the @doc annotation handling in the Dotty macros
• publish all dependencies for Dotty and remove the withDottyCompat override from the dependencies

---

~@lihaoyi and @anatoliykmetyuk feel free to give this a preliminary review if you'd like. I'll remove the WIP label once the first two items are fixed.~ The PR is ready for review now. ~Before CI can correctly test this however, scala-requests (https://github.com/lihaoyi/requests-scala/pull/68) and upickle (https://github.com/lihaoyi/upickle/pull/322) must be published for Dotty.~

[jodersky]

The latest commit fixes the ambiguity error. It unfortunately requires a change to upickle, as shown here.

[anatoliykmetyuk]

Both upickle and requests are on maven now, so it should be possible to test this against the CI.

[jodersky]

Perfect, thanks for your help @anatoliykmetyuk. I noticed that some test now fails. I'll check what's up

[jodersky]

@lihaoyi, @anatoliykmetyuk, the build is green! Let me know what you think

[lihaoyi]

@jodersky I haven't looked at this in detail, but if all the tests pass I say ship it

[jodersky]

:pray: thanks