RFR: 8254880: ZGC: Let ZList iterators be alias templates

[Per Liden]

Since we can now use C++11/14 features, I propose we simplify the ZList iterators by making them alias templates.

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Commit messages:
 - 8254880: ZGC: Let ZList iterators be alias templates

Changes: https://git.openjdk.java.net/jdk/pull/695/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=695&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254880
  Stats: 33 lines in 2 files changed: 0 ins; 30 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/695.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/695/head:pull/695

PR: https://git.openjdk.java.net/jdk/pull/695