Add yang.model.api.EffectiveStatementInference

A number of places need to exchange a stack of EffectiveStatements as a means to maintain cohesiveness within a single logical operation on top of an EffectiveModelContext. While the details and semantics of exchange can vary, we need an overarching construct, the basis of which is:

• associated EffectiveModelContext

• an ordered sequalve of EffectiveStatements

Introduce EffectiveStatementInference as such a capture and integrate it with SchemaInferenceStack, providing a simple means of exchanging SchemaInferenceStack state.