Improve parameters passed in Rust-C bindings

[zoedberg]

currently all complex objects like arrays and dictionaries are serialized into strings in order to be easily passed from C to Rust.
this could imply some performance issues so let's investigate a better way to pass complex objects and therefore remove all JSON serialization

[zoedberg]

after some satisfying results reached while trying to get consigment bytes (for #21), I think I could tackle this issue

[zoedberg]

In order to remove JSON serialization, I see 2 possible solutions:

• mapping dictionaries (or whatever key-value structure is available on that language) into C structs
• mapping classes (or whatever complex structure is available on that language) into C structs

The first solution should require a little less code, while the second should give more flexibility.
I personally prefer the second one, but I would like to hear your opinion and possibly other solutions that didn't come in my mind.

To be clearer, I'll do an example (code taken from node.js demo):

const config = {
    network: "testnet",
    stash_endpoint: "lnpz:/tmp/rgb-node/testnet/stashd.rpc",
    contract_endpoints: {
        Fungible: "lnpz:/tmp/rgb-node/testnet/fungibled.rpc"
    },
    threaded: false,
    datadir: "/tmp/rgb-node/"
}

Currently we call start_rgb like this:

start_rgb(config.network, config.stash_endpoint, JSON.stringify(config.contract_endpoints), config.threaded, config.datadir)

We could improve code to get config.contract_endpoints (removing JSON stringify) or we could create a Config class in the native code. Both would require adding code on the swig interface to convert these structures into C structs.

So using the first solution, the call would change into:

start_rgb(config.network, config.stash_endpoint, config.contract_endpoints, config.threaded, config.datadir)

And, with the second solution, into:

config = new Config(config.network, config.stash_endpoint, config.contract_endpoints, config.threaded, config.datadir)
start_rgb(config)

[dr-orlovsky]

I also think the second approach is more natural for OOP languages. Also with it we may have stuff like config = Config::default() or config = new Config() where the structure is initialized with some defaults

[zoedberg]

@dr-orlovsky since we've postponed issue removal, should I remove JSON serialization also from there? currently the rust method takes a single string that gets parsed into a IssueArgs struct by serde