Add getChars to CharSequence

[Martin Buchholz]

I've often wished that CharSequence had getChars methods, as many of the
concrete implementations already do.
In jdk8 with default methods, this is possible!
This will make some of the String code a little nicer and more efficient.

Here's a preliminary patch in this direction, that overlaps with some of
the work done in

6206780: (str) Forwarding append methods in String{Buffer,Builder} are
inconsistent
Summary: update StringBuilder & StringBuffer to consistently handle
forwarding to AbstractStringBuilder. Some additional cleanup (removal of
refs to sub-classes from AbstractStringBuilder)

http://cr.openjdk.java.net/~martin/webrevs/openjdk8/getChars/

If we have consensus that this is a good idea, I can flesh this out.