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

Run all dhal-lang typecheck tests

Run all dhall-lang tests for normalization

Pass all import tests

Environment variable failure can fallback

Allow imports to be either binary or source form

Update to new UnionType selection semantics

Fix rubocop except for parser metrics

Working parser

Keep around type information when we have it

fix type => [value|element]_type

Refactor LetBlock, allow single LetIn