Tests are long for good reasons
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
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.
Implement many builtins and do Optional section
Start to implement normalization rules
Decoding Dhall from binary works