Constant style::sharing::SHARING_CACHE_SIZE

pub const SHARING_CACHE_SIZE: usize = 31

The amount of nodes that the style sharing candidate cache should hold at most. We'd somewhat like 32, but ArrayDeque only implements certain backing store sizes. A cache size of 32 would mean a backing store of 33, but that's not an implemented size: we can do 32 or 40.

The cache size was chosen by measuring style sharing and resulting performance on a few pages; sizes up to about 32 were giving good sharing improvements (e.g. 3x fewer styles having to be resolved than at size 8) and slight performance improvements. Sizes larger than 32 haven't really been tested.