Try on generic visitor

Builtins are part of the standard set

Fix unit tests

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

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

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

Remove swap files