* 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
* complete transition from MacOS#version to MacOS#release
throughout backend code
* recast related constants and comments as "release" instead
of "version", and "point release" instead of "full version"
* accepts a Boolean value
* replaces the never-documented `postflight` methods `enable_accessibility_access` /
`disable_accessibility_access` (`remove_accessibility_access`)
* declarative: unlike the `postflight` approach, does not have to be
invoked in two places
* de-document obsolete form `caveats` / `assistive_devices`
* bump Travis seed to avoid false test failures