Fix occasional panic in FrequencySketch in debug build

[tatsuya6502]

Fixes #267.
Fixes #113.

This PR prevents an internal index_of method of FrequencySketch from panicking by arithmetic overflow.

• Only debug build was affected by this issue.
• Release build was not affected unless overflow checks were explicitly enabled at the build time.

Changes

• Replace an u64 addition (+) operator in FrequencySketch::index_of method with wrapping_add method.
  • Wrapping (modular) addition is the desired behavior for index_of method.
  • The + operator will behave the same to wrapping _add method in release build (where overflow checks are disabled).
• Add test harnesses (test cases) for Kani Verifier to ensure there is no chance for arithmetic overflows and other panics to occur in FrequencySketch (not only for index_of method but also other methods).
  • Enable a CI job to run Kani test on every git pushes.

Verification

• Ran cargo kani before fixing the issue, and arithmetic overflow was detected.
• Ran cargo kani after fixing, and no issue was detected.

[tatsuya6502]

Merging.