Rust-Lightning, not Rusty's Lightning!

Currently somewhere near 5% towards usable, published to see if there is any real interest from folks in either contributing to or using a lightning rust library.

The goal is to provide a full-featured but also incredibly flexible lightning implementation, allowing the user to decide how they wish to use it. With that in mind, everything should be exposed via simple, composable APIs. The user should be able to decide whether they wish to use their own threading/execution models, allowing usage inside of existing library architectures, or allow us to handle that for them. Same goes with network connections - if the user wishes to use their own networking stack, they should be able to do so! This all means that we should provide simple external interfaces which allow the user to drive all execution, while implementing sample execution drivers that create a full-featured lightning daemon by default.

For security reasons, do not add new dependencies. Really do not add new non-optional/non-test/non-library dependencies. Really really do not add dependencies with dependencies. Do convince Andrew to cut down dependency usage in rust-bitcoin.

Assorted random TODOs:

• Create a general timer interface - this should be passed around in reference form to most objects to allow them to register functions which are called on a timer. By default we should provide an implementation of this which uses some newfangled rusty promise-y library, but should generally ensure a client can simply integrate this into whatever existing timer interface they use.

• Networking...having a simple bytes-in-bytes-out interface which does message handling and calls our encryption layer is probably the right approach. We should then also probably use the same promise-y library we use for timers to do socket selection and reading/writing.

• Figure out how to expose when-to-connect and who-to-connect-to.

• Implement when-to-connect and who-to-connect-to based on route/node rumoring and channelmanager state.

• Some kind of serialization format for on-disk storage of things like channels, channelmonitors, routing db, etc.

• BOLT 10/network bootstrapping implementation.

• Some kind of DoS thing including ban tracking and putting that info in HandleError (and also rename HandleError) to be propagated up...and then handled.

• All the random TODOs and unimplemented!()s across the codebase.

• BOLT 11 (invoice/address creation/generation) implementation

• Type-ify our somewhat random usage of Uint256/[u8; 32]. Use Sha256dHash where appropriate, create our own types for everything else.

• Some kind of logging subsystem/API.

Notes on coding style:

• Use tabs. If you want to align lines, use spaces. Any desired alignment should display fine at any tab-length display setting.

License is Apache-2.0.