RFR: 8301599: Serial: Refactor nested closures in DefNewGeneration

Wed Feb 1 10:20:30 UTC 2023

Simple refactoring broken into two commits:

1. Rename `KeepAliveClosure`.
2. Extract and move closures to cpp file.

Test: hotspot_gc

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Commit messages:
 - move
 - rename

Changes: https://git.openjdk.org/jdk/pull/12359/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12359&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301599
  Stats: 124 lines in 3 files changed: 39 ins; 59 del; 26 mod
  Patch: https://git.openjdk.org/jdk/pull/12359.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12359/head:pull/12359

PR: https://git.openjdk.org/jdk/pull/12359