Move as much code out of node into core as possible

[gavofyork]

This a meta-issue. Offenders:

• node_consensus: Should be removed to core.
• node_network: Should be removed to core.
• node_transaction_pool: A general, runtime-pluggable transaction queue as in Generic Transaction Queue logic in runtime #728 should allow this to be eliminated.
• node_api: The pub mod api { impl_stubs!( ... ) } should be able to be reused for creating an ergonomic calling interface in native code, thereby eliminating the need for most of the API boilerplate.
• node_service: Looks pretty boilerplatey. Use macros and/or more generics in order to get it down to the essentials.
• Runtime refactorings:
  • DigestItem decl should be part of macro #926
  • Automatically impl deposit_event  #920

[bkchr]

I'm started working on the consensus stuff.

[gavofyork]

@gnunicorn & @bkchr could you check with each other that your changes don't collide? :)

[bkchr]

@gavofyork I think this issue can be closed?