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

to_s before load

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

as_dhall for Symbol

Parse expression from ENV var import

Instead of the path-only support from before.

Standard reader requires UTF-8

To disable heuristics, force remote content to be UTF-8 text.

Fail fast when encoding is bad