Improve a substatement context lookup in yang statement parser

Very often we need to check whether a substatement with a specific statement definition exists under some statement context in yang statement parser or to look up a substatement (or substatements) with a specific statement definition.

Currently, in order to do that we need to iterate over all current context's substatements and check whether there is a substatement we are looking for.

I think using a MultiMap keyed by StatementDefinition instead of List for substatement contexts will improve these lookups significantly.