Leverage Immer diffing

[arximboldi]

Consider a scenario:

lager::reader<immer::map<K, V>> parent = ...;
lager::reader<V> child1 = paren[k0];
lager::reader<V> child2 = paren[k1];
...
lager::reader<V> childN = parent[kN];

At the moment, whenever any value changes inside the map, even if it is only the element pointed at by one of the children, there will be N comparisons performed when propagating the data to the children (each child has to check if their element did change).

However, Immer now supports strucutural-sharing aware diffing.

This means we can implement an optimization in Lager such that it detects this particular scenario and uses immer::diff() to detect which children did change and propagate changes directly to those, ignoring the children that did not change, potentially turning O(N) into O(1) for most common UI patterns!