(avatars) display names

[benjaminbollen]

set a display name on the Circle node, which is not unique across the graph, but can be used to make more human-readable URLs.

the display name would be composed out of two parts: <display>-<unique-base58-nonce>
where the display is editable on-chain, but the contract should check for length and valid alphanumeric characters upon setting

and the unique nonce, should be contract derived, should take a sufficient length for uniqueness (it can be made to be unique);
consider whether this should be part of {EIP20-name()} as it can be mutable; or a separate display(); or even relegated to an external name service contract