Key changes:

• Ruby 2.4 support.
• Annotations removed from Object, added to RDL::Annotate.
• Annotations also accessible via RDL module and RDL::RDLAnnotate
  modules; see README.
• o.type_cast(t) now must be written RDL.type_cast(o, t) and
  similarly with instantiate! and deinstantiate!.