Refactor builtins

The fill/unfill code is much simpler now

IntegrityCheck stores raw hash internally

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

Facility to flatten let blocks

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

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

Timeout mechanism

Sets timeouts in HTTP readers, checks for deadline exceeded before each
resolution step, and uses Timeout::timeout for pure-ruby computations.

Fix Typo

Dhall::Text is obviously the Text builtin, not the Double one.

Do not load if already loaded

In case the resolver has some values already

Update handling of builtins

Always parse to a special value and not to variables.
Encode seperately.

Normalize text concat as interpolation

Parse expression from ENV var import

Instead of the path-only support from before.

Add grab-bag of useful utils onto expressions

Add as_dhall refinement

Optional#value is not what we want here