Improve documentation and code clarity of yang-parser-impl

YANG Parser implementation is one of the oldest and most core
parts of YANGTools and MD-SAL, over the time codebase grew by
lot of bugfixes, which left us with harder-to-read code base.

This enhancement is to preserve public-facing functionality of yang-parser-impl
but improve internal documentation and code to make it easier to understand
for other developers outside core team.