[javalin]

Got some good feedback here last time, so posting again. I rewrote the library to Kotlin, but I had to leave a few things java, mainly the SAM interfaces and the Javalin class iteself. This had to be done due to limitations in Kotlin (https://youtrack.jetbrains.com/issue/KT-14151, https://devnet.jetbrains.com/thread/461516)

The rest of the library will be ported to Kotlin if this issue is resolved, or and alternative solution is discovered.

I guess the biggest change for Kotlin devs is that Request and Response are now Kotlin.

Feedback/criticism is welcome!