8173893: JShell: reduce memory leaks
Reviewed-by: jlahoda

8171981: JShell: Fails compilation: new Object().getClass().getSuperclass()
Reviewed-by: mcimadamore

8156101: JShell SPI: Provide a pluggable execution control SPI
Reviewed-by: jlahoda

8134254: JShell API/tool: REPL for Java into JDK9
Summary: Adding the implementation of the jshell (read-eval-print-loop) tool.
Reviewed-by: briangoetz, mcimadamore, psandoz, forax
Contributed-by: robert.field@oracle.com, bitterfoxc@gmail.com, jan.lahoda@oracle.com