YANG Tools / yang: code improvements and bug fixing

Continuous task for code improvements and bug fixing for components under yang directory:

yang-binding
yang-common
yang-data-api
yang-data-impl
yang-data-util
yang-maven-plugin
yang-maven-plugin-it
yang-maven-plugin-spi
yang-model-api
yang-model-util
yang-parser-api
yang-parser-impl