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

Fix Proc decode

Hoist single-label enum into array

Split Union and Enum

Stop putting special case conditionals for Union#value.nil? everywhere
and just split the classes.

Customizable cache

Caches implemented in this commit:

StandardFileCache - cache conforming to dhall-lang
RamCache - stores in a hash, meant for single-load caching
NoCache - when you want to be sure