recast method `destination_path` as `staged_path`

* part of DSL 1.0 review
* `destination_path` was always a bit vague (it refers to the Cask-specific, version-specific location under  `/opt/homebrew-cask/Caskroom`)
* here renamed `staged_path` to match upcoming command verb `brew cask stage`
* rename also intended to reduce confusion when we implement copying as a configurable alternative to symlinking
* transitional `destination_path` methods to remain while Casks are converted (this was documented as a part of the DSL, and used by 39 Casks in main repo)
* unrelated variables containing "stage" recast for clarity