This reverts commit 21fb15e,
because:

- There was no motivation for this upgrade (nothing specified
in the commit message).
- It might introduce bugs that we don't want to deal with, at
the moment.
- We might revert this revert shortly after finishing our
milestone4 (closing channels).

This is a fix for changes introduced in
joemphilips#118 which caused
chain hashes to get serialized in reverse order.

A lot of types in DotNetLightning are defined as discriminated unions
with a single constructor with the same name as the type. eg:

    type NodeId = | NodeId PubKey

This commit redefines these types as:

    type NodeId(id: PubKey) = ...

The main motivation for this is that having types and constructors with
the same name breaks the json serialization library on older mono
versions. It can also lead to confusing error messages. Besides that,
it's also somewhat pointless to define a type as a discriminated union
but then only give it one variant.

This commit adds the JsonMarshalling module which defines json
serialization converters for a bunch of types exported by
DotNetLightning. This allows the consumer of DotNetLightning to
serialize the channel state to a wallet file.