Consider dropping xtask in favor of just, make or scripts

[nickgerace]

Related to #265, we should consider using a task runner like just and dropping the xtask sub-crate.

This isn't a big priority, but given xtask's low discoverability and the proposal that #265, we have a chance to flatten the repository into one crate.

[nickgerace]

@uncenter wanted to tag you in this as well in case you had any thoughts!

[uncenter]

Sounds good to me, I'm a big fan of just scripts as well.

[nickgerace]

Closed by #271