Fix for Ruby 2.6+

Ruby 2.6 added a new method named `then` to `Object` which broke us using the
method_missing based DSL.  Switch to def_attr for then/else to fix this.
Update to dhall-lang v10.0.0
Switch encoding of binary URI format
Support mixed-kind records
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