Kotlin `master` (3.0.0-SNAPSHOT) builds again (tests passing too)

[Tatu Saloranta]

Quick note on Kotlin module: similar to most other Jackson components,
its "master" branch (default one when cloning) is for next major
Jackson version, 3.0.0. Most PRs should not be done against it but
either 2.10 (for smaller patches, bug fixes) or 2.11 (next minor
version: for things that change API or are riskier changes).

Due to major changes in Jackson core components, Kotlin module has not
been building for `master` in a while. But I finally figured out how
to fix everything and it does build, passing the test suite same as
2.x. I will try to keep it building from now on, to be in sync with
core component changes.

-+ Tatu +-

ps. Lots of good PRs for Kotlin module, which is great! But still
looking for help with maintenance.