[thorwhalen]

Closes #158

Adds dol.path_of(store, key): the file-system path of key in a file-backed store, however wrapped. It follows every layer's _id_of_key (the existing inner_most_key walk) to the innermost FileSysCollection, validates the result there (a key leaving rootdir raises KeyValidationError, a KeyError), and raises TypeError when the core is not a file store. Nothing is read or created.

Design note: a free function rather than a Files.path_of method, deliberately. Store.__getattr__ forwards unknown attributes to the inner store with the outer key unmapped (the #83 trap), so a method propagated through wrap_kvs would silently return a wrong path. The function needs no propagation, works for any stacking, and is purely additive. xdol can default its path_of= to it.

Tests: dol/tests/test_filesys.py::test_path_of_follows_wrappers_and_validates + doctests.