Improve XCM fuzzer (#6190)

* Add improved XCM fuzzer

* Add command for running a single input

* Add installation command

* Fix @m-cat's nit

* Add newline

Co-authored-by: Kian Paimani <[email protected]>

* Add info about current directory in fuzzing README

* Update Cargo.lock

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