Self-describe CBOR for Expression#to_binary

CBOR.encode(expression) and Expression#to_cbor remain untagged "standard
Dhall" but the default Expression#to_binary now always tags the output
with self-describe CBOR.  This is supported by the standard for decoding
the output will still be compatible with all compliant implementations,
and it makes the binary format easier to identify in the wild.

Allow Symbol in yaml-to-dhall

Add experimental json/yaml to dhall scripts