Allow unannotated format string tokens

Update to latest dhall-lang

Fix expectations for tests

Refactor projection typecheck

Multihash prefix on filename

Switch to URI

This could be a literal, so may be frozen

New header format

Update parser

Support \u{XXX} and ban surrogates

implement as Location and use URI internally

Tests can be long and procedure like

Record projection by expression / type

Do not commute multiplication when normalizing

Self-describe CBOR for Expression#to_binary

CBOR.encode(expression) and Expression#to_cbor remain untagged "standard
Dhall" but the default Expression#to_binary now always tags the output
with self-describe CBOR.  This is supported by the standard for decoding
the output will still be compatible with all compliant implementations,
and it makes the binary format easier to identify in the wild.

Fix lint

Headers is probably a Dhall::List

But we map it to strings... just use `to_a` since we're not staing in
Dhall-land anyway.

A whitespace-only last line is still an indent

RecordProjection.for

Fix for #join on empty list