convert to Kotlin

[kaushikgopal]

Instacart is deeply committed to Kotlin. Also we've noticed that having the code base would just encourage more folks to be on top of the library.

Throwing this in here, to gauge feedback. We'd probably keep a tag for posterity in case there are folks who strongly prefer to keep the java version

[fahadayaz]

Coroutines support would be great :)

[kaushikgopal]

we just merged 4.0.0.alpha. Kotlin + coroutines 💪🏽