Refactor `identity_did`'s `DID` trait

[UMR1352]

Description

Refactor trait DID and CoreDID, removing the need for DID types (e.g. IotaDID, DIDJwk, DIDKey) to implement AsRef<CoreDID>.

Motivation

The DID: AsRef<CoreDID> constraint forces implementing types to encapsulate a CoreDID, making the type's layout more complex. If this wasn't the case a did-jwk would be represented simply as:

pub struct DidJwk(Jwk);

instead of:

pub struct DidJwk {
  did: CoreDID,
  jwk: Jwk,
}