RFR 8164277: JShell API: Snippets are immutable and should be available for post-mortem analysis

[Robert Field]

Please review...

Bug:
     https://bugs.openjdk.java.net/browse/JDK-8164277

Webrev:
     http://cr.openjdk.java.net/~rfield/8164277v0.webrev/

Specdiff:
http://cr.openjdk.java.net/~rfield/8164277v0.specdiff/jdk/jshell/JShell.html

API:
http://cr.openjdk.java.net/~rfield/8164277v0.jshellAPI/jdk/jshell/JShell.html

Thanks,
Robert