Generalize and publish the Assyst command parser

[trueharuu]

This should move all base parser functionality (proc macros, ParseArgument trait, and others) into a separate, more general crate that then could be used by others.

[Jacherr]

This will probably be done as part of a larger refactor of assyst-core to consolidate it down to several library crates, since assyst-core has inflated massively and become quite unwieldy and does too much for a single crate. I'll open a separate issue at some point to figure out what needs moving and where. Not sure about publishing and how that would be handled yet.