Update to latest dhall-lang

Fix for substitute on LetBlock/LetIn

Hoist single-label enum into array

Remove empty re-opens from normalize

Normalize text concat as interpolation

Add grab-bag of useful utils onto expressions

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