Convert `RelationLeaper` trait into `Relation` methods

[IFcoltransG]

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.