Cleanup the README text
Adapt targets in build.xml to Ant naming conventions

Also reformat the section related to Upsource
Refactor building of kotlin-runtime.jar in build.xml

- use the "new-kotlinc" macro to compile Java sources (can be disabled via an
  optional argument) as well as Kotlin
- bend over backwards not to repeat source paths passed to kotlinc and javac
- split "runtime" task into several smaller ones, which makes it possible to
  rebuild (and repack the whole jar) constituent modules separately
Fix path duplication in "kotlin-for-upsource" in build.xml
Drop ForTestCompileRuntime

Because the process of building kotlin-runtime.jar will likely get more
complicated soon, and that logic will already be present in build.xml.

Run "ant compiler-quick runtime" instead before All Tests