Perform incremental compilation using official compiler API

Investigate support for incremental compilation using the [official Kotlin compiler API](https://github.com/JetBrains/kotlin/tree/master/compiler/incremental-compilation-impl) instead of the mechanism currently used ([as described in the README](https://github.com/fwcd/KotlinLanguageServer/blob/master/README.md#this-repository-needs-your-help)), because this might improve performance considerably.

Start by reading the README section linked in the issue to understand the current compilation mechanism, then review the official Kotlin compiler incremental-compilation implementation. Determine how the existing approach could be replaced and how performance should be evaluated; the work is complete when incremental compilation uses the official API and its impact is assessed.

Written by the indexing model from the issue text.