Run all dhall-lang tests for normalization

Pass all import tests

Update to new UnionType selection semantics

Fix rubocop except for parser metrics

Working parser

fix type => [value|element]_type

Refactor LetBlock, allow single LetIn

Clean up style violations

Remove multi-argument applications

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

Working typechecker

Try on generic visitor

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.

Update to fixed tests

Bare minimum refactor to pass the metrics

Clean up style

Pass most compliance tests

No import or multiline string support yet

Beta normalization complete

Implement "shift" function for variable indices

All beta normalization except for functions