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
Default max import depth of 50

So even without a timeout, there is a limit on bonkers cases.

Standard resolver defaults to INIFINTY since the standard does not
define a depth limit.
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.
Update to dhall-lang 7.0.0
Document Dhall::Coder
Dhall::Coder

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

Dhall::Text is obviously the Text builtin, not the Double one.
Allow as_dhall to be customised by encode_with

Dhall has a goal to replace YAML.  Ruby's ecosystem already has deep
integration with YAML (specifically with the Psych implementation in
stdlib) -- so if an object customises its YAML encoding, but doesn't
know about Dhall, customise the Dhall encoding in a compatible way.
Do not deep-annotate OpenStruct#as_dhall
Encoding binary string as List Natural

Certainly better than an error, and gives the dhall side access to the
individual bytes.
Do not load if already loaded

In case the resolver has some values already
Remove hardcoded list of builtin names
Allow Symbol in yaml-to-dhall
Update handling of builtins

Always parse to a special value and not to variables.
Encode seperately.
Remove empty re-opens from normalize
Normalize text concat as interpolation
largeExpression works now, but is slow

So only run in CI
Update dhall-lang and work with the new ABNF
Next