RFR 8160127: JShell API: extract abstract JDI and abstract streaming implementations of ExecutionControl

[Robert Field]

Please review --

Bugs:

     JShell API: extract abstract JDI and abstract streaming 
implementations of ExecutionControl
     https://bugs.openjdk.java.net/browse/JDK-8160127

     JShell API: Reorganize execution support code into 
jdk.jshell.execution (previously sent for review, and combined here)
     https://bugs.openjdk.java.net/browse/JDK-8159935

Parent bug:

     https://bugs.openjdk.java.net/browse/JDK-8159118

Webrev:

     http://cr.openjdk.java.net/~rfield/8160127v0.webrev/

API:

     http://cr.openjdk.java.net/~rfield/jshell_8160127/

     Specifically --
http://cr.openjdk.java.net/~rfield/jshell_8160127/jdk/jshell/execution/package-summary.html

Thanks,
Robert