Store layer statistics in a layer file

[matko]

There's some layer statistics (like number of triples, number of nodes/predicates/values, layer stack size), which are recursively calculated by traversing a layer stack. We could also store these statistics in a file, saving us the computation.

As an example, this would be useful to have for cases where we need to decide among multiple equivalent layers, which of them is best to use for querying. At present, we have to recompute the relevant statistics each time we need them.