dynamic type name references

[mccanne]

Currently, types that don't resolve at compile-time cause a fatal error (except for single-name references like type(foo) that uses expr.TypeFunc). We should extend expr.TypeFunc to be able to resolve any and all type names recursively defined in a nested type and such resolution should adapt to changes in the typedef.

Along these lines, we should update the ZSON spec to allow typedef changes and update the runtime to continue to operate in the face of new bindings.