Add explicit cradle predicates and multi cradle depend on its cradles

[fendor]

This changes multi-cradles to have the name stack or cabal if their hie.yaml specification solely consists of stack or cabal and none cradles. To make stuff easier, we add is*Cradle predicates to the API. Users of this library can make use of them to obtain the cradle-type after it has been parsed.

Fixes haskell/haskell-ide-engine#1490

Also, changes the ActionName to be an extensible ADT.

[fendor]

cc @cocreature, @ndmitchell
This patch would change the type of Cradle to Cradle a and actionName :: String changes to ActionName a. For you the main difference would be that you would have to change occurrences of either Cradle or ActionName to Cradle Void and ActionName Void respectively, afaict. Would you be fine with this PR, or do you have counter arguments, or other suggestions?
Based on this PR, I plan a follow up PR for hooking into the parsing of hie.yaml, e.g. extending Config to allow specifying a custom cradle.

[ndmitchell]

Doesn't seem too big a deal for the changes in ghcide.