Use tags for extensibility

[casey]

Currently, the etching is one or two trailing varints in the runestone, which are the rune, and a small amount of additional metadata. This is simple, but limits us to a small amount of metadata, and gives us limited extensibility. We might want additional metadata, or some way to extend or modify the protocol in the future, and using a separate push for the etching would make that easy.

[casey]

We could also use a similar approach to inscriptions:

• Runestone is a sequence of varints.
• Parse as sequence of (tag, value) fields.
• If an unrecognized even tag is encountered, then all input runes are burned, and if there is an etching, its supply is set to zero.
• A tag of 0 means that the remaining varints are edicts. If we've previously encountered an unrecognized tag, then the edicts are ignored and all runes are burned.

This increases the size of the runestone by one byte, if there are fields, by one byte per field.

If we do this, we need to emphasize that ALL tags are reserved for the protocol, and should not be used with prior coordination.

[casey]

Things we cannot do:

• We cannot miss any etchings. If we have unique runes, then in order to avoid conflicts, un-upgraded and upgraded implementations must recognize the same runeset.