refactor: enhance MFU cache implementation and documentation (#72)

* refactor: enhance MFU cache implementation and documentation

Updated the `MfuCore` implementation to utilize a `HeapEntry` struct for managing frequency and sequence number, improving eviction logic and maintaining O(log n) complexity. Enhanced documentation for clarity, including details on thread safety and the new `HeapEntry` structure. Added iterator methods for accessing key-value pairs, keys, and values, improving usability and alignment with Rust API guidelines.

* docs: improve MFU cache documentation formatting

Updated the MFU cache documentation to enhance clarity by correcting formatting for `HeapEntry` references. This change aligns with Rustdoc standards and improves the overall readability of the implementation notes and core structure descriptions.

Author: TKorr