Refactor library and binary crate

[ralexstokes]

the previous repo had a library crate containing common code mixed with a "builder" crate

first change: refactor the common crate as mev-lib from the distinct builder crate

second change: move the command line utility from the top-level mev-rs crate into a separate binary crate

now that the work has been done I see I could have just left the mev-rs crate alone (I was thinking mev-lib would become mev-rs but the names will clash b/t binary crate and library crate) but the work is already done and this does lay the foundation for future command line utilities in the mev-rs ecosystem that could be distinct from running the various node types (e.g. data utilities?)