Run dhall-lang binary-decode tests

Decode unions the same way as_dhall makes them

Use Dhall names for everything that encodes untagged

Disambiguate hoisted union tags properly

An array may contain two nomnatively-equivalent types who are
structurally different -- so they need different (but related) union
tags since they will not share a type.

Enum should not have itself in the alts

More useful to_s for AST nodes

Normalize list element types before comparing

Facility to flatten let blocks

Prefer double quotes for strings

Update and run latest alpha-normalization tests

Initial dhall-compile script

Add to build that after tests pass we compile the whole Prelude

Fix for stack level too deep on long comment

Allow typechecking filled builtins

Add new useful facilities to Function

as_dhall for Proc

Cannot actually serialize this, but can be useful for passing functions
in to Dhall functions, etc.

Update to latest dhall-lang

Handle self-describe tag anywhere

Fix for substitute on LetBlock/LetIn

Improve the multiline algorithm

Require that the whitespace being stripped is a common prefix.

Allow tabs and blank lines in multiline strings