Add xcm fuzzer (#3869)

* Extend xcm-simulator with a fuzzer for xcm exec

This commit adds a very basic fuzzer that fuzzes the xcm exectution,
in particular the process_instruction function.

* Update cargo.toml in xcm-simulator-fuzzer

* Add xcm-fuzzer to honggfuzz workflow

* Update Cargo.lock

* Update xcm/xcm-simulator/fuzzer/Cargo.toml so honggfuzz shows up on top

Co-authored-by: Keith Yeung <[email protected]>

* Update relay_chain.rs in xcm-fuzzer

* Use MAX_XCM_DECODE_DEPTH instead of hardcoded decode limit in xcm-fuzzer

* Add comment on how to generate coverage report in xcm-fuzzer

* fix warnings, fmt, and unused result

* fix compiler

Co-authored-by: Vincent Ulitzsch <[email protected]>
Co-authored-by: Vincent Ulitzsch <[email protected]>
Co-authored-by: Shawn Tabrizi <[email protected]>
Co-authored-by: Keith Yeung <[email protected]>