Support Equivalent trait for the cache key type K

[tatsuya6502]

Summary

Support the Equivalent trait from the equivalent crate for the cache key type K. Replace all usages of the trait bound K: Borrow<Q> with Q: Equivalent<K>.

For example,

Before

pub fn get<Q>(&self, key: &Q) -> Option<V>
where
    K: Borrow<Q>,
    Q: Hash + Eq + ?Sized,

After

pub fn get<Q>(&self, key: &Q) -> Option<V>
where
    Q: Equivalent<K> + Hash + ?Sized,

https://github.com/indexmap-rs/equivalent/blob/master/README.md

These traits are not used by the maps in the standard library, but they may add more flexibility in third-party map implementations, especially in situations where a strict K: Borrow<Q> relationship is not available.

Changes

• Add the equivalent crate to the dependencies.
• Reexport the equivalent module.
• Replace all trait bound K: Borrow<Q> with Q: Equivalent<K>.

Relates to #166

[codecov]

Codecov Report

Attention: Patch coverage is 92.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 94.17%. Comparing base (e9bcd86) to head (fd68e80).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #492      +/-   ##
==========================================
- Coverage   94.31%   94.17%   -0.15%     
==========================================
  Files          43       43              
  Lines       20492    20433      -59     
==========================================
- Hits        19328    19242      -86     
- Misses       1164     1191      +27     

[tatsuya6502]

Merging.