Add fuzzer for the compact custom codec implementation from PR #6720 (#7091)

* Add fuzzer for the compact custom codec implementation introduced in PR #6720.

This commit adds a fuzzing harness for the custom compact encoding/decoding
introduced in PR #6720.

* Update primitives/npos-elections/fuzzer/src/compact.rs

Co-authored-by: Bastian Köcher <[email protected]>

* Update Cargo.lock: Add changes in elections-fuzzer

* Change indentation from spaces to tabs

Co-authored-by: Vincent Ulitzsch <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>