Add proxy object for java.lang.String implementations

The trait scala.scalajs.runtime.RuntimeString contains all
implementations for methods on java.lang.String. The compiler
automatically forwards calls to these methods to the trait's
implementation class.

This also fixes #164 (Implement java.lang.String.getChars)
Forward constructor on String to RuntimeString (equivalent to methods)

This also fixes #154 (Implement all String constructors)