Fix bug that prevented writer from accepting Math::Int128 objects

The Serializer worked fine, but the Tree::InMemory object's use of JSON caused
it blow up when it saw an object in the data associated with a subnet.
Use Sereal in the Serializer as well - no need for both JSON & Sereal
Make bigint test pass with Math::Int128 0.07 and 0.09
Remove unneeded block

This is a remnant from when we used the bigint pragma.