Allow imports to be either binary or source form

Update to latest dhall-lang

Add cache_key test runner

Anything we'll eventually run is a skip

Fix test to new binary format

No more null fragment

Use upstreamed type tests

Working parser

Simplify binary tests for now

Fix style in tests

Forgot to add this file

Refactor Optional/None typecheck

Refactor Function annotate

Run tests per docs

Use path like names in more tests

Implement integrity checks

Working typechecker

Type inference/checking about half done

Passing unit tests for features implemented so far

Support headers for HTTP import

Binary encoding

Every AST node uses as_json to express the generic structure for
encoding itself.  Expression aliases as_cbor to as_json by default and
then uses as_cbor in to_cbor (aliased as to_binary).

This design is because we need to override the encoding of Double for
CBOR, but want to keep the normal Float value for as_json in case that
is used directly (to avoid surprising the user).

If trying to import from IPFS, fallback to gateway