Add missing comparison ops to js.String and fix compiler to support them
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)
Merge pull request #201 from gzm0/runtime-string

Forward methods on java.lang.String to dedicated implementation objects