Decouple Predicate from implementations

[epage]

assert_cli is going to optionally allow users to create predicates for verifying the results of their application.

My current thoughts are

• Allow users to decide if they want predicates and, if so, which
• Minimize predicate API breakages from impacting assert_cli's API

From this, my thought is that we should move Predicate into a predicates-core crate. assert_cli will accept a predicates_core::Predicate. Users of assert_cli can then depend on predicates if they want and pass some in. This will allow us to make breaking changes to the predicates without breaking the APIs that accept predicates.