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

Speed up tests by not resolving when not needed

Test everything the README says

Change list of unit tests

to_a keeps elements as Dhall

Clarify that the README is the authority

Fix version

Add build instructions to README

aspell

Gemify

Downgrade to GPL