RFR: 8281879: Serial: Merge CardGeneration into TenuredGeneration

[Albert Mingkun Yang]

Mostly mechanical changes; `git diff --color-moved=zebra` probably helps to see the actual (small) change, such as dropping the `virtual` modifier, introducing small helper methods to avoid naming collision, etc.

Test: tier1-6

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Commit messages:
 - flatten-cardgen

Changes: https://git.openjdk.java.net/jdk/pull/7482/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7482&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281879
  Stats: 893 lines in 14 files changed: 350 ins; 526 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7482.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7482/head:pull/7482

PR: https://git.openjdk.java.net/jdk/pull/7482