Reimplement libcache purging in GSGrid

[foxostro]

NSCache / libcache has a mechanism to purge cache contents on memory pressure. Unfortunately, libcache behaves poorly under contention, so I had to get rid of it. Reimplement in GSGrid, somehow. Maybe just have GSGrid maintain a fixed number of items in its cache, or measure app rprvt. In any case, it won't be as elegant as libcache's mechanism, but that's not using a publicly documented API.