Cadence 0.0.20

Issue by Issue

• Finalize error handling. #117. #116. #116. #105.
• Implement conditional catch. #110.
• Test that first error is returned to outer callback. #114.
• Implement finalizers. #111.
• Test that a fixup is not called with an error. #111.
• Implement try and catch functions. #109.
• Remove shifted callbacks. #112.
• Upgrade to Proof 0.0.30. #106.