std.http: add http server

* extract http protocol into protocol.zig, as it is shared between client and server
* coalesce Request and Response back into Client.zig, they don't contain
  any large chunks of code anymore
* http.Server is implemented as basic as possible, a simple example below:

```zig
fn handler(res: *Server.Response) !void {
    while (true) {
        defer res.reset();

        try res.waitForCompleteHead();
        res.headers.transfer_encoding = .{ .content_length = 14 };
        res.headers.connection = res.request.headers.connection;
        try res.sendResponseHead();
        _ = try res.write("Hello, World!\n");

        if (res.connection.closing) break;
    }
}

pub fn main() !void {
    var server = Server.init(std.heap.page_allocator, .{ .reuse_address = true });
    defer server.deinit();

    try server.listen(try net.Address.parseIp("127.0.0.1", 8080));

    while (true) {
        const res = try server.accept(.{ .dynamic = 8192 });

        const thread = try std.Thread.spawn(.{}, handler, .{res});
        thread.detach();
    }
}
```

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