Interrupts are possible during any stage of program execution. Certain
areas are protected with Hbc::Utils.ignore_interrupts, but for the rest
we should exit gracefully without spitting out an ugly stack trace.
* 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