* YANG Leafref feature update

* Closer adherence to RFC 7950. Some of this is changed behavior, some is new feature.
  * Essentially instead of looking at the referring leaf, context is referred(target) node
  * Validation uses referred node
    * Validation changed to use type of referred node, instead of just "string"
  * Auto-cli
    * Changed to use type of referred node for typecheck
    * Completion uses referred node
  * Required instance / less strict validation
    * New: Leafrefs must refer to existing data leaf ONLY IF YANG `required-instance` is true
    * Previous: All leafrefs must refer to existing data leaf node
* Fixed: [Autocli does not offer completions for leafref to identityref #254](https://github.com/clicon/clixon/issues/254)

test/test_leafref.sh

@@ -47,11 +47,13 @@ module example{
              description "Absolute references existing interfaces in if module";
              type leafref {
                  path "/if:interfaces/if:interface/if:name";
+		 require-instance true;
              }
          }
          leaf relname {
              type leafref {
                  path "../../if:interfaces/if:interface/if:name";
+		 require-instance true;		 		 
              }
          }
          leaf address {
@@ -59,12 +61,14 @@ module example{
              type leafref {
                  path "../../if:interfaces/if:interface[if:name = current()/../relname]"
                     + "/ip:ipv4/ip:address/ip:ip";
+		 require-instance true;
             }
          }
          leaf wrong {
              description "References leading nowhere in yang";
              type leafref {
                  path "/ip:interfaces/ip:interface/ip:name";
+		 require-instance true;
              }
          }
     }
@@ -76,6 +80,7 @@ module example{
         leaf template{
             type leafref{
                 path "/sender/name";
+		require-instance true;	
             }
         }
     }