EIP-2253: Add wallet_getAddressBook JSON-RPC method

[danfinlay]

@Arachnid

The idea with clef is to separate APIs that are necessarily part of a node - fetching chain state and submitting transactions - from APIs that are concerned with user accounts - signing, key management etc.

This was also the motivation of the wallet_ prefix, as suggested elsewhere, and in use by this proposal.

What would be the extra steps that make this very-clef-like?