RFR 8186089: Move Arena to its own header file

[harold seigel]

Hi,

Please review this JDK-10 change to move class Arena into its own header 
file for reasons described in the bug.

Note that since the SA doesn't use class Chunk, it need no longer be 
exported to it.  Also, class Chunk is in the new arena.hpp header file 
(as opposed to just the .cpp file) because class HandleMark uses it in 
handles.hpp.

Open Webrev: 
http://cr.openjdk.java.net/~hseigel/bug_8186089/webrev/index.html

JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8186089

The change was tested with the JCK Lang and VM tests, the JTreg hotspot, 
java/io, java/lang, java/util, and other tests, the co-located NSK 
tests, JPRT, and with RBT tier2 - tier5 tests.

Thanks, Harold