Kotlin 1.4

[thecodewarrior]

Kotlin 1.4 offers several useful improvements for LibLib:

• SAM conversions for Kotlin interfaces means we no longer need to mix Kotlin and Java for any custom SAM interfaces.
• The new type inference algorithm means we can use the unqualified method { block } syntax for Java SAM interfaces, meaning we no longer need to have separate Kotlin methods that forward to the ones with SAM interfaces.
• -Xjvm-default=all means we don't have to constantly annotate interface members with @JvmDefault.
• Explicit API mode forces us to be more deliberate about what is exposed in the public API. This will be the bulk of the changes in this PR. (this feature singlehandedly took the number of warnings up from 63 to 3,723)

[thecodewarrior]

Merging this before Kottle for 1.4 is released. For now, just add the Kottle-2.0.0-slim.jar file to the runtime mods directory.