Implement DocumentedNode methods in terms of EffectiveStatement

Most of DocumentedNode/SchemaNode methods can be implemented in terms of EffectiveStatement lookup. Provide default implementations which defer to the EffectiveStatement view provided by YANGTOOLS-1119. We already have the interfaces to achieve these in yang.model.spi.EffectiveStatementMixins.

Most of them (except the withFlags() stuff) needs to be migrated to yang.model.api. There they need to be tied with EffectiveStatementEquivalent and then integrated into individual SchemaNode definitions.