Introduce a Store

[sekur]

A Store should house a collection of Models and serve as the root for XPATH accessibility tree (see #31).

Currently, the Yang schema instances are captured by the Yang class. This enables schema-level resolutions during Yang.parse.

The Store will need to attach the Model properties and allow population of data state. The Store will be the primary entity for data provider (persistence layer) adaptors and support load/dump type operations (see #24).

We still want to preserve direct per-Model interactions. Since a Model does not need to be a YANG module schema, we will need to differentiate "module" models vs non-module models and handle them appropriately from the Store.

[sekur]

verified integration with yang-express. still has planned missing features but the introduction looks to be functional.