RFR: 8147599: [JVMCI] simplify code installation interface

[Roland Schatz]

Hi,

Please review this change to the JVMCI code installation interface:

webrev: http://cr.openjdk.java.net/~rschatz/JDK-8147599/webrev.00/
jira: https://bugs.openjdk.java.net/browse/JDK-8147599

The new classes in the jdk.vm.ci.code.site package used to be inner 
classes in the removed CompilationResult class, no actual code changes 
there.

Thanks,
Roland