Add getChars to CharSequence

[Martin Buchholz]

Another version of this change is ready.  No longer preliminary.  Tests
have been written.
http://cr.openjdk.java.net/~martin/webrevs/openjdk8/getChars/

This kind of change is particularly subject to feature creep, and I am
trying to restrain myself.

I even addressed some of Ulf's suggestions.
The "Msg" suffix  is gone.
I reverted changes to AbstractStringBuilder.replace.
I kept the naming convention for getChars parameter names.
Parameter names and exception details continue to be maddeningly
unsystematic, but they should be a little better than before.