This commit adds a ChannelState struct which is used to track the
policy and state of a channel in the *outgoing* direction. This will
be used to check forwards against the node's advertised policy and
track the movement of outgoing HTLCs through the channel.

Note that we choose to implement this state *unidirectionally*, so a
single channel will be represented by two ChannelState structs (one in
each direction).

Add a single representation of a simulated lightning channel which
uses our state representation to add and remove htlcs. For simplicity,
each side of the channel is represented as a separate state, with the
only interaction between the two through the changes to local balance
that happen when we settle htlcs.

Add an implementation of the LightningNode trait that represents
the underlying lightning node. This implementation is intentionally
kept simple, depending on some SimNetwork trait to handle the mechanics
of actually simulating the flow of payments through a simulated graph.

We want to be able to distinguish between expected and critical payment
errors in our simulated payments. An expected error occurs due to a
lightning-related failure (such as running out of liquidity), and a
critical one happens because something has gone wrong with our
simulator (for example, an assertion failure about balances).

This commit adds an some utilities to ForwardingError to make this
distinction and display errors properly.

Add an implementation of our SimNetwork trait that will do the heavy
lifting of propagating htlcs through a simulated network.