Timeout mechanism

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

Update text-or-binary heuristic

The `cbor` gem is a bit janky and it turns out catching NoMemoryError is
not safe, so use an encoding-based heuristic to detect binary vs utf8
data.

Dhall::Coder

Full "to basic ruby types" deserializing, set up as a drop-in for
ActiveRecord::Base#serialize

to_s before load

Fail fast when encoding is bad

Add a Dhall.dump

Add Dhall.load

Add as_dhall refinement

Pass all import tests

Allow imports to be either binary or source form

Working parser

First pass at import resolution

Builtins are part of the standard set

Start to implement normalization rules

Decoding Dhall from binary works