[crac] RFR: 8352408: [CRaC] Minor CRE API improvements

[Timofei Pushkin]

Minor fixups for CRE API:

`crexec` is also adopted to conform to the restore data API docs change.

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Commit messages:
 - Improve CRE interface

Changes: https://git.openjdk.org/crac/pull/214/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=214&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352408
  Stats: 22 lines in 3 files changed: 7 ins; 2 del; 13 mod
  Patch: https://git.openjdk.org/crac/pull/214.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/214/head:pull/214

PR: https://git.openjdk.org/crac/pull/214