Support multiway-if expression

[Futarimiti]

Add support for MultiWayIf.

Examples:

multi_if :: Int
multi_if = if | e -> 1
>>>>>>>>>>>>>>| o -> 3

handle :: IOError -> IO ()
handle e = putStrLn $ if | isAlreadyExistsError e -> "File already exists"
>>>>>>>>>>>>>>>>>>>>>>>>>| isDoesNotExistError e -> "File does not exist"
>>>>>>>>>>>>>>>>>>>>>>>>>| isEOFError e -> "Reached EOF"
>>>>>>>>>>>>>>>>>>>>>>>>>| isPermissionError e -> "File access denied"

[Futarimiti]

Not sure under which directory should I add tests, guard/ or if/