Handle usize overflow on big cache capacity

[tatsuya6502]

This PR fixes the issue #25, usize multiply overflow on big cache capacity.

Changes

• Removed the multiply operation (max_capacity * 32) when setting the capacity of the popularity estimator FrequencySketch.
  • The implementation of the estimator was replaced in an older version of Moka and the multiply operation was not really needed since then.
• Added code to ensure that the capacity of the popularity estimator always fits within a range of 128u32..=u32::MAX.
• Changed the type of the internal fields of the estimator from usize to u32 so that the estimator will work consistency on any platform with different pointer width.

Verification

• Added unit tests to sync::base_cache and unsync::cache modules to ensure overflows should not happen and sizes of the population estimators are correct:
  • Before applying the fix, ran the tests with the debug profile and confirmed they panicked as expected:

    ---- sync::base_cache::tests::test_skt_capacity_will_not_overflow stdout ----
    thread 'sync::base_cache::tests::test_skt_capacity_will_not_overflow' panicked at
    'attempt to multiply with overflow', src/sync/base_cache.rs:395:39
    

  • After applying the fix, ran the tests again and verified they passed. PASSED
• Ran some mokabench workloads on Moka before/after the fix and verified that hit rates and run-time durations did not change. PASSED

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Fixes #25

[tatsuya6502]

Merging.