Refactor RecordProjection typecheck

Refactor Union typecheck

Refactor some of typecheck

Get closer to fitting the metrics

Clean up style violations

Implement integrity checks

Always encode OptionalNone in new style

Flatten/unfill applications and builtins when encoding

Remove multi-argument applications

They only exist in binary encoding, so let's keep them there.

Working typechecker

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

First pass at import resolution

Try on generic visitor

Fix unit tests

Generic subexpression_map

Make every Expression a ValueSemantics and use the known list of
attributes to map generically over any AST node.

Introduces a conditional, but removes boilerplate from every single
class.

Migrate to dhall-langv6.0.0

Bare minimum refactor to pass the metrics

Clean up style

Pass most compliance tests

No import or multiline string support yet