Add proptools.TypeEqual

When appending properties, it may be necessary to determine if two
property structs are the same "type".  A simple Go type comparison is
not sufficient, as there may be interface{} values in the property
structs that contain different types.  Add proptools.TypeEqual that
returns true if they have equal types and all embedded pointers to
structs and interfaces to pointers to structs have the same nilitude and
type.