Hoist single-label enum into array

Split Union and Enum

Stop putting special case conditionals for Union#value.nil? everywhere
and just split the classes.

Encoding binary string as List Natural

Certainly better than an error, and gives the dhall side access to the
individual bytes.

Update handling of builtins

Always parse to a special value and not to variables.
Encode seperately.

as_dhall for Symbol

Add as_dhall refinement