RFR: 7903328: Introduce a new method 'clear' in interface 'Multiset'

[Guoxiong Li]

Hi all,

This patch adds a method named `clear` in interface `Multset` and implements the method in its sub-class. A corresponding test case is added and all the test passed locally.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - CODETOOLS-7903328

Changes: https://git.openjdk.org/jmh/pull/81/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=81&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903328
  Stats: 75 lines in 4 files changed: 72 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jmh/pull/81.diff
  Fetch: git fetch https://git.openjdk.org/jmh pull/81/head:pull/81

PR: https://git.openjdk.org/jmh/pull/81