Remove version tag from encoded expressions

[Gabriella439]

... as standardized in dhall-lang/dhall-lang#362

[f-f]

@Gabriel439 I felt there was a more beautiful way to write that :)

Now the CBOR decoding from cache goes fine, but in case you're importing a hash-protected expression, and the hash has been generated by a previous implementation, then there's a HashMismatch happening.
I think it's thrown here because the hash of the term given by the new encode doesn't match the hash in the file (as the CBOR terms are different).

The last bit of backwards-compat could be another alternative here where instead of only one actualHash, we have an alternative in which we try to hashExpression (that calls the new Dhall.Binary.encode), and to hashExpressionLegacy (that would call a Dhall.Binary.encodeLegacy, which would wrap the term with the version in the same way we do currently on master) for every past standard version (or the last one, or the last two, etc), so that there would be at least one hash that matches.

[f-f]

@Gabriel439 I tested this branch locally with spago and it works nicely. Very many thanks! 😊

[Gabriella439]

@f-f: You're welcome! Don't forget to review the corresponding change to the standard 🙂