Add build instructions to README
Add grab-bag of useful utils onto expressions
Add experimental json/yaml to dhall scripts
Fix to_cbor usage
nil hash values are omitted from record
Add as_dhall refinement
Test typechecker edge cases
Optional#value is not what we want here
Update makefile with more helpers
Run all dhal-lang typecheck tests
Run all dhall-last tests for cache_key
Run all dhall-lang tests for normalization
Pass all import tests