Leverage Code Contracts properly

[shiftkey]

We've been using this here and there but I'd really like get consistent with these and save some of my sanity:

TODO:

• confirm this and this are all we need for devs to get started.
• investigate options for runtime checks (it can generate it into a separate assembly)
• see whether other options like netfx-guard or Fody.NullGuard are more suitable