Normalizer & validation rework, improved spec conformity

• Existence of fields, arguments, directives, ... is now correctly done during normalisation.
• Improved field merging - added validation of response shape.
• Added new interface TypeConditionable which is applied to composite + outputable + named types.
• Added exception for unknown type/directive.
• Added exception for variable type which is not inputable.
• Added exception for fragment type condition which is not TypeConditionable.