Add AssignmentParserResult type alias
squash! Replace SimpleCommandParserImpl with SimpleCommandParserBase
fixup! Replace SimpleCommandParserImpl with SimpleCommandParserBase
Replace AssignmentParserImpl with AssignmentParserBase

This refactoring commit removes the AssignmentParserImpl class template
in favor of the new AssignmentParserBase abstract class. Using a class
template only for class dependency injection seems to be an overkill. An
abstract class is sufficient and more readable.