RFR 8156101: JShell SPI: Provide a pluggable execution control SPI

[Robert Field]

Please review the new Service Provider Interface for using alternate 
execution implements.

Bug:
     https://bugs.openjdk.java.net/browse/JDK-8156101

Webrevs:
     http://cr.openjdk.java.net/~rfield/8156101v0_lang.webrev/
     http://cr.openjdk.java.net/~rfield/8156101v0_ws.webrev/

Javadoc:
     http://cr.openjdk.java.net/~rfield/jshell_spi/

Note: The SPI is tested since it is used by the default JDI 
implementation (which has no special access).  However, tests using an 
alternative execution engine will be included.

Thanks,
Robert