Equivalents of QueryableBlock::{len, iter} for namespaces

[ggutoski]

Like #991, #931 except for namespaces. Should be easy given what we've already done for those issues. Indeed, we should probably factor out the namespace-specific code from whatever code we write for those issues and re-use it here.

Suggested names, but feel free to think of something better:

• len -> num_namespaces
• iter -> namespace_iter

As per EspressoSystems/hotshot-query-service#328 (comment) these new methods will not be added upstream to QueryableBlock. Instead just add them to the concrete BlockPayload struct in this repo. (Currently that's in block2.rs along with everything else VID.)