Split up yang-data-impl

yang-data-impl currently hosts a large number of classes, some of which are related, some of which are not.

There are multiple components we want to split out:

• SchemaTracker should live in yang-data-util
• InMemoryDataTree should live in yang-tree-impl (or simple)
• Baseline abstract classes and similar should live in yang-data-spi
• Immutable nodes and their builders should live in yang-data-immutable

This may also require splitting up yang-data-api as well.