RFR: 6588: Move JMC core to JDK 8

[Marcus Hirt]

WIP. Now that core requires JDK 8, we can make it a little bit more easygoing when using streams. I'm thinking it would be nice to remove the need for the StreamModel altogether as well.

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Commit messages:
 - Adding values()
 - Supporting streaming in IItemIterable
 - Additional cleanup
 - Inlining stream
 - First move
 - 6588: Initial cleanup

Changes: https://git.openjdk.java.net/jmc/pull/194/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=194&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6588
  Stats: 961 lines in 71 files changed: 488 ins; 411 del; 62 mod
  Patch: https://git.openjdk.java.net/jmc/pull/194.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/194/head:pull/194

PR: https://git.openjdk.java.net/jmc/pull/194