Implement depends_on :cask

[tapeinosyne]

This PR includes library and (some) test code for hard Cask dependencies, i.e. Casks which must always be installed prior to the depender.

There are some warts. Most notably:

• An invasive optional argument, skip_cask_deps, is passed down from Cask::Installer.install to satisfy_dependencies, in order to avoid redundant loading and (re)sorting of Cask subdependencies.

Secondarily:

• TopologicalHash, a helper class for dependency sorting, should not be in the top namespace. We do have extend, but it is reserved for monkeypatches.
• CaskDependencies should also not be in the top namespace, being a specialized class.

[rolandwalker]

Warts or not, it is a much-wanted feature, and looks mergeable.

My only question after reading is why everything happens in the constructor. (Not that it matters in practice, because you read the dependencies in the next line after instantiation.)

[tapeinosyne]

No reason other than overlap and utility. A naked array of first-level dependencies is already available at the DSL level as depends_on.cask, and CaskDependencies serves no purpose beyond sorting. (Perhaps the name is insufficiently descriptive of the current implementation.)

If desired, I have no qualm with refactoring the class into a more conventional structure.

[rolandwalker]

Just curious, no need to bother with refactoring.

The name is also perfectly descriptive, if you think of a CaskDependencies instance as a collection. Purely for entertainment value, it is worth noting that one can make it act more like a collection by include Enumerable or just providing an each method, after which

deps.sorted.each do |dep_token|

could be spelled without the sorted

deps.each do |dep_token|