Add link_identity_short workaround

[Kailai-Wang]

Context

As topic.

	// a hotfix to work around the "too long encoded call size" problem
	// it's similiar to `link_identity`, but with reduced parameters:
	// - the first identity is the prime identity, whose signature is required
	// - the second identity is the to-be-linked identity
	// in this sense, it can only be called via DI