v6.3.0: Efficiency improvements, argument checking and error messaging

✨ Major features and improvements

• NEW: Add thinc.check module to specify argument constraints for functions and methods.
• NEW: Add thinc.exceptions module with custom exception messaging.
• Add LSUV initialisation.
• Add averaged parameters, for reduced hyper-parameter sensitivity.
• Improve efficiency of maxout, window extraction and dropout.

📋 Tests

• Reorganise and improve tests.
• Reach 100% coverage over the entire package.