Hbc is the namespace, Hbc::Cask is the object representing a Cask
One step on the path to reducing the surface area of the God-object
we've grown over the years. :)
* convert existing Cask:: namespace to Hbc::
* move Homebrew-fork code under Hbc::
* move freestanding classes such as Tty and TopologicalHash under Hbc::
* recast HOMEBREW_CASK_ constants as HBC_
* modify our Homebrew Formula for backward compatibility
* devscripts and dev docs