API redesign

[Tishka17]

• load/dump vs parsers/serializers - fix naming
• dynamic name mapping
• "ORM mode" or custom gettersm which also required for multidict
• use one class as fields list, but create another or parse different class
• per-field schema, not only per type
• type groups and plugin support
• splify getting parsers from schema
• other methods like jsonschema, object update, etc

[zhPavel]

We should remove support for Optional without arguments