Unnamed unions WIP.  About to change MemberInfo format, which will require re-bootstrapping the code generator.

Pull TextBlob out of capnpc-capnp and into KJ as kj::StringTree.  Use it in the stringification code to make that code cleaner and avoid using iostreams.

Allow SchemaLoader to use a callback to lazily load schema nodes, including dependencies of loaded nodes.

Improve const-correctness of readers and builders.  We enforce transitive constness, meaning you should not be able to construct a mutable Builder from a const Builder (you can always convert them to Readers instead).