Parse expression from ENV var import

Instead of the path-only support from before.
Standard reader requires UTF-8

To disable heuristics, force remote content to be UTF-8 text.
Fail fast when encoding is bad
Speed up tests by not resolving when not needed
Test everything the README says
Change list of unit tests
to_a keeps elements as Dhall
Clarify that the README is the authority
Add build instructions to README
Downgrade to GPL
Add grab-bag of useful utils onto expressions
Add a Dhall.dump
Add experimental json/yaml to dhall scripts
Fix to_cbor usage
nil hash values are omitted from record