Bug fixes, code improvements and optimizations

[arterialist]

Bug fixes

• fixed null cell unpacking in get_auction_info. The last_bid nullability condition was breaking the method, making one unable to get the auction info

Optimizations

• replaced a number of widely used common functions' modificators to inline_ref
• potential save of ~500 gas on command unpacking function in case of invalid input data
• potential save of 26 gas in change_dns_record if the auction has active bids

Code improvements

• typos and spelling fixes in readme and comments
• removed unused msg_value argument of the unwrap_signed_cmd
• replaced hardcoded error codes with constants
• minor refactoring and code readability adjustments
• replaced all unused variables with _, making it easier to spot potentially unnecessary unpacking chains which can be avoided by adjusting the data scheme, storing some of the data in the same type, etc.