API revamp — iterators not callbacks

[fengb]

const client = zCord.Client.init(auth);
const gateway = try client.startGateway(.{
    .allocator = &gpa.allocator,
    .intents = .{},
    .auto_reconnect = true,
});
defer gateway.destroy();

while (true) {
    switch (try gateway.recvEvent()) {
        .connect => {},
        .dispatch => |dispatch| {
            // dispatch.seq, dispatch.name, dispatch.data
        },
    }
}

Advantages:

• clear separation of concerns — Client.init() captures only the auth, startGateway() stores the connection info (instead of awkwardly sticking it onto the client)
• no callbacks, no generics!

Disadvantages:

• ws.recvEvent() is horribly non-concurrent — this was the main reason I preferred callbacks since it controlled the event lifecycle
• normalizing events might take a bit of effort — connect is especially tricky since we consume a part of the data, so unwind might be a requirement

[fengb]

Minor disadvantage:

• Handling individual events still triggers a lot of errors. The proper way to do it would most likely be splitting into a separate function simpler try... which is basically a callback. But at least the function context is controlled.

Disadvantage averted:

• Converted recvEvent() to be more resource like. Now it holds onto a "lock" that needs to be deinitted, where attempting to recv multiple consecutive events crash spectacularly. This should hopefully be a lot more obvious that it cannot be done, instead of being a UB footgun.