Allow self-describing CBOR

Switch to registry pattern for TypeChecker.for

If trying to import from IPFS, fallback to gateway

Tests are long for good reasons

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.

Alpha normalization

Internal flag to disable for acceptance tests from the standard (which
assume beta-only normalization in some cases).  Otherwise, just always
alpha normalize as we go.

Clean up style

Implement many builtins and do Optional section

Start to implement normalization rules

Decoding Dhall from binary works