Change the JSON API function to be from a serialised struct

This is the first experiment towards using structs to define the API.
It adds it as a new method on the existing Chat struct.

The types in this struct could improve as well as many other things.
But this is a straight forward translation of the existing json! macro
into a struct.

deltachat-ffi/Cargo.toml

@@ -21,6 +21,7 @@ libc = "0.2"
 human-panic = "1.0.1"
 num-traits = "0.2.6"
 failure = "0.1.6"
+serde_json = "1.0"
 
 [features]
 default = ["vendored", "nightly", "ringbuf"]