Add method to get XPath path in primary instance with index predicate only for nodes bound to repeats

[lognaturel]

TreeReference.toString essentially returns an XPath path. However, that path includes index predicates for every node, even if no disambiguation is ever needed.

In the case of the primary instance, index predicates are only ever needed for repeats.

This functionality is probably best provided by FormIndex which represents the position of a node in the primary instance. It has both the raw path of the node and information to know which node in the path corresponds to a repeat.

More context at getodk/collect#3126. https://github.com/opendatakit/collect/pull/3231/files#diff-b25f62214ddaa8161b77f62ab938dcaeR288 is probably the implementation needed, it just needs a good name.