split client out further into more digestable parts

[matthewpeterkort]

Make this repo make sense and be maintainable by:

• Reorganizing the code into smaller packages / files
• Simplifying interfaces to only implement core functions
• Moving side/non-core functions into struct method implementations
• Factor g3cmd business logic out of util.go into organized upload and download packages
• Cleanup request, function interface implementations
• Move most constant definitions into common/constants.go
• Convert small functions that are only called once into closures
• Delete functions that don't need to be functions or have a near duplicate functions that do almost the same thing