When a CAS node has tags, ocas get includes them in the envelope's
value as a `tags` array. When a variable's value hash has tags,
ocas var get includes them as a `valueTags` array (separate from
the variable's own tags/labels). Untagged nodes/values produce
byte-identical output as before (no empty array serialized).
Schemas @ocas/output/get and @ocas/output/var-get extended to
accept the new optional fields.
Closes#53
xiaoju
4ebae73257