Convert RelationLeaper trait into Relation methods #53
Description
The RelationLeaper trait is implemented by Relation and has some extra methods for constructing leapers. However, it would be simpler if they were defined directly on the struct rather than a trait plus its implementation.
The trait doesn't look necessary because it's only implemented by Relation, and intra-doc links don't currently work for implementations of a trait. In #52 I linked to the trait methods, but it would have been cleaner to link directly to Relation's methods.