simpler declarative `build`

[nmushegian]

Based on our current usage it seems like we can just have a single pure build function. (We should also still have stateful builder pattern).

const pack = dpack.build({
  network: hh.network.name,
  types: {
    'Gem': {
      artifact: hh.artifacts.readArtifact('Gem')
    },
  },
  objects: {
    'gemfab': {
      typename: 'Gem',
      address: gem.address,
      artifact: hh.artifacts.readArtifact('Gem'),
      alsoPackType: true
    }
  }
})

notice:

• we can allow omitting redundant typename and objectname in places it can be inferred from the key. In later versions these could be distinct, but here they can't be
• alsoPackType can be patched onto the object directly