Move readline out of jruby.jar.

There are a number of reasons for this:

* Allows us to version it separately (potentially even moving it
  out of JRuby repo altogether.
* Resolves issues where our bundling or our boot classloading
  interferes with parts of jline or jansi. I have removed the
  hack we had in place that prevented jansi from erroring while
  breaking several readline features (like tab completion,
  JRUBY-6996) on Windows.
* Reduces jruby.jar size for cases where you don't need readline.

This also follows our (and MRI's) pattern of moving ext libraries
out of core into gems.