Pass all import tests

Allow imports to be either binary or source form

Anything we'll eventually run is a skip

Fix test to new binary format

No more null fragment

Implement integrity checks

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

Run all tests on CI

First pass at import resolution