Do not use private Sun API in preloader
Drop "+StdLib" from manifest entries
Use findLoadedClass instead of hand-written cache in ParentLastURLClassLoader
Minor style fixes in ParentLastURLClassLoader
Avoid redefining kotlin2js task in "ant jslib"

This helps to specify custom runtime for the newly built kotlin2js (which is
supposed to be different for the two bootstrapping steps). Also now it's not
required to launch ant-tools task before jslib to build JS library
Minor, rename JS Ant tasks

To be able to use the build file from the terminal conveniently, make names
shorter and avoid autocompletion conflict with "compiler"
Minor, fix style issues in ant tasks