use keys for events to deal with vertical structure (chords)

[polymorphicengine]

currently it is not possible to regard the stack construct as an alternative to lists, since too much information is lost in the transformation to signals, for example a lookup operator wouldn't be possible:

"[1,2 3,4]" !!! 2 --> "2 3"

there is just no way of knowing that the events corresponding to value 2 and 3 respectively "belong together".

my suggestion is to add a new field in the Event type representing this information, i'm thinking of the type

data Key = Key [Int]

where the list of integers represents the path in the "stacktree" (for dealing with nested stacks). for example the keys in

"[1 2, [3, 4 5], 6" would be

1 -> [0]
2 -> [0]
3 -> [1,0]
4 -> [1,1]
5 -> [1,1]
6 -> [2]

since tidal doesn't have many functions dealing with vertical structures it is a pretty easy addition, i think only overlay and stack should be really affected, which can be defined as:

overlay :: Pattern a -> Pattern a -> Pattern a
overlay p p' = Pattern $ \st -> map (\e -> e {key = addKey 0 (key e)}) (query p st) ++ map (\e -> e {key = addKey 1 (key e)}) (query p' st)

stack :: [Pattern a] -> Pattern a
stack xs = Pattern $ \st -> concatMap (\i -> map (\e -> e {key = addKey i (key e)}) $ query (xs!!i) st ) [0..length xs - 1]

i made a working branch of the additions (without tying the changes into the parser) here:

https://github.com/polymorphicengine/Tidal/tree/1.9-stack-try

let me know what you think! i will happily add these changes to the current tidal 2.0 branch if everyone agrees