fixing bug with OptimizedDiff between boolean types

fixing failed ygen unit tests

fix code generation to contain enum vals from YANG

OptimizedDiff: Dropping scalar leafs set to their default vals

OptimizedDiff to return leaf paths

introducing ygot.OptimizedDiff

setting GO version to 1.18 for static analysis

uncommenting code to validate duplicate keys in list during unmarshal

option to tolerate missing nodes in SetNode

supporting PartialKeyMatch for SetNode