Synchronous did:key resolver

[vdods]

I'd like to suggest that DIDKey could be made into a synchronous DID resolver, since that particular resolver doesn't have any need for async.

This could either simply factoring the body of the DIDKey::resolve method out into a impl DIDKey pub fn resolve_sync, or more formally by making a DIDResolverSynchronous trait, have DIDKey implement that, and add (into ssi/src/did_resolve.rs) an automatically derived impl of DIDResolver for anything impl'ing DIDResolverSynchronous.

Thoughts?