Automatic reference_hash calculation at compile-time

[encody]

Standards like NEP-141 have a field called reference_hash, described as:

the base64-encoded sha256 hash of the JSON file contained in the reference field.

With compile-time macros, it is possible to automatically download the reference field and calculate the hash. This would be immensely convenient for quickly adding an additional layer of security. Currently, not very many fungible tokens use the reference, and therefore the reference_hash fields, but the ones that do use reference don't seem to use reference_hash. This may be due to projects wanting to keep non-volatile metadata on-chain and volatile metadata off-chain, where it's easier to maintain control of and update.

Regardless, this would be a really cool feature to have.