path FEATURE new compiled path structure (#1108)

Refactoring includes using it for
instance-identifier and for checking leafref,
it is evaluated using stanrad XPath. Predicates
used for lyd_new_list2(), tests included.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 80c5ae3..c3d1f1b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -206,6 +206,7 @@
     src/log.c
     src/hash_table.c
     src/set.c
+    src/path.c
     src/context.c
     src/tree_data.c
     src/tree_data_free.c