Fix for new Set#=== in newer Ruby

Because breaking things is point releases is how Ruby rolls.
Update dhall-lang
Pass newly added tests
Minitest fail fast for unit tests
Update dhall-lang
Simplify Natural/subtract on x === y
Implement dependent types, assert : A === B
Update dhall-lang
LetBlock is just sugar now and not part of the AST
Update dhall-lang

This includes the new record selection inspection optimizations.
Some test setup tweaks in dhall-lang
Fix alpha normalization on normalize tests
Normalize multiline string literal line endings
Add Natural/subtract
Fix stack level too deep for single quote string
Specialize the visitor since we only use it one place
Do not start IO work until asked
Update resolve tests to test actually valid cases
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