interledger-stream API review for v1.0-rc

[gakonst]

interledger-stream public items marked with x are documented. Non-exported items are not taken into account. Follows format from #561.

Part of #557.

• StreamDelivery (struct)
• send_money (fn)
• SendMoneyFuture (struct)
  • Not public, but its implementation and fields must be documented in detail since it's our main low-level payments protocol and may be a prime target for future optimizations etc.
• Error (enum)
  • Do we need this? It's only imported in the SPSP Error wrapper enum, but that variant does not seem to get hit ever
• ConnectionGenerator (struct)
• `PaymentNotification (struct)
• StreamNotificationStore (trait)
• StreamReceiverService (struct)

[bstrie]

• Error: not sure if this was already addressed in the error overhaul, but if we don't need it then let's get rid of it

[gakonst]

Not sure why I had written this bullet initially, the error type is actually used within stream