bring this crate into conformance with compiler-team crate policy

[nikomatsakis]

The compiler team is working on a standard policy around its external crates (see rust-lang/compiler-team#19) -- we need to bring this crate into conformance, though the policy is not yet finalized, so it's not 100% clear what this means.

[Cypher1]

Hey @nikomatsakis looks like rust-lang/compiler-team#19 has been closed a while.

Can you share any requirements from it for this crate?

Context: I'm doing some Rust + Logic programming work at the moment and am trying to find some time to contribute back so am looking for starter issues.