Feature/create stub

[mbwgh]

This PR allows the user to create a stub to satisfy a typed hole, for example for a scenario described here.

Example:

process :: M.Map String Int -> Bool
process = M.foldl' _ False

can be converted to

process :: M.Map String Int -> Bool
process = M.foldl' go False

go :: Bool -> Int -> Bool
go = undefined

via an Intention Action (i.e. by pressing alt+Enter when the cursor is over the typed hole).

[rikvdkleij]

@mbwgh Thanks!!