add "depth" parameter to position

[yenicelik]

    pub depth: u32,

if we add a depth parameter, we should be able to easily created nested strategies. not sure how to reflect this on the UI side as of now, but good to keep this in the backend anyways.

basically depth of 0 indicates that the cranks should run on these things first. this initial crank-run will create LP tokens, for example.

then we can run the cranks for depth=1, using the LP tokens generated from the cranks with depth=0, etc.