[WIP] Updates to ilp lib

[adrianhopebailie]

@d1no007 @austin-king @karzak @kincaidoneil @emschwartz

These are the updates I was talking about on the call today.

I have tried to organize things a little better.

• Defined a type hierarchy and provided both types and then also functions to (de)serialize between them and JSON for SPSP message purposes.
• This way there is the concept of a Receiver which is a stateful invoice that wraps a STREAM server and SPSP servers become a specialization of that.
• The express extension is an approach to easily hosting an SPSP server....
• I started defining some functions that made sense for different use cases in protocol.ts
• I created a concept of a client that is what ultimately led me to trying to implement net.socket over STREAM and to these experiements: https://github.com/adrianhopebailie/ilp/tree/master/experiments

I recall now why I needed to figure out protocol for the sender and receiver to communicate over STREAM; it was to negotiate things like the payee details etc that we use SPSP for today.

i.e. Assuming you have a secured STREAM connection (you could have exchange addresses and secrets via some other channel) how do you now exchange all the other info like an invoice or requested amount.

STREAM doesn't make it easy to send a fixed receive amount to the recipient so this protocol could be used to set that up (figure out the current rate and an allowable amount of slippage once you start streaming).

Hope some of this is helpful, a lot is a bit icky and unfinished