Improve lifetime of string reference returned by DepTriple::relation

[danieldk]

The lifetime of the string reference returned by DepTriple::relation is bound to DepTriple. This is ok when DepTriple owns a String. However, if the string is borrowed, a more appropriate lifetime would be the actual lifetime of the string reference.

This makes the life easier for users of DepTriple, because now something like depgraph.head(idx).unwrap().relation() will complain that the relation outlives the triple.