Complete subexpression detection

[lognaturel]

No description provided.

[eyelidlessness]

Just a couple quick notes for clarity of what I think this issue covers:

• The intent of subexpression detection (and related functionality) thus far is to establish dependency relationships between a given form instance node and other aspects of the form
• This includes, but is not limited to, location path references resolving to other instance nodes
• Already-implemented examples going beyond references to other instance nodes include:
  • Translations (calls to jr:itext()) establish a dependency on the active language
  • Itemsets (when calls to instance()) establish a dependency on the referenced secondary instance (and the data referenced therein); these may also include predicates (choice filters in XLSForms lingo) which establish dependencies to primary instance nodes

This last point is potentially a beneficiary of #17 (or other potential solutions to the same motivating problem): if an instance() expression includes a predicate with an absolute reference to a repeat descendant (as many of the existing fixtures I'm familiar with do), we'll need to be able to contextualize those dependencies appropriately.