RFR 8176412: jshell tool: automatic imports are excluded on /reload causing it to fail

[Robert Field]

Imports "automatically" added by <fix-shortcut> i or <fix-shortcut> v 
(where <fix-shortcut> is usually Alt-F1 or Alt-Enter) are not added to 
the history, and thus aren't reloaded on /reload or /env.
This causes such /reload or /env to fail with errors.
Fix by having the eval go through higher level entry points in the 
JShellTool.  Make the low-level JShell state private to the tool, by 
adding an access point for the one remaining state access "stop()".
Side-effects:
   In the unlikely/impossible case that the import fails, an error will 
be displayed (which is better than silent failure).
   If the user defines a feedback mode that gives feedback on import, 
the feedback will be displayed on auto-import.  This works fine (I have 
tested) since the line is redrawn.

Please review....

Bug:

     https://bugs.openjdk.java.net/browse/JDK-8176412

Webrev:

     http://cr.openjdk.java.net/~rfield/8176412v0.webrev/

Thanks,
Robert