Tests are long for good reasons
Try on generic visitor
Builtins are part of the standard set
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
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
Update to latest binary format
Run all tests in CI
Fix binary tests
Migrate to dhall-langv6.0.0
Need dhall for build
Use git-extras for `git root` in build
Re-gen build files
Bare minimum refactor to pass the metrics
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