code combinator without loss of performance

code combinator works by combinating reusable operators, and it functions by callback instead of value passing via channels(e.g. actix), so no performance loss.

current operators:

1. mysql
2. sql_runner
3. http_api
4. http_server
5. wasm
6. simple_auth
7. saga_aggregator

Each operator has its own configurations, defined in struct Config in specific file under core/src/operator.

For example, if the input json is:

{
    "operators": [
        {
            "kind": "mysql",
            "config": {
                "dsn": "mysql://user:pass@172.168.3.1:3307/db?readTimeout=3s&charset=utf8mb4"
            }
        },
        {
            "kind": "sql_runner",
            "config": {
                "sql": "select * from user limit 10"
            }
        },
        {
            "kind": "http_api",
            "config": {
                "uri": "/",
                "method": "GET"
            }
        },
        {
            "kind": "http_server",
            "config": {
                "listen_addr": "127.0.0.1:8088"
            }
        }
    ],
    "applies": {
        "1": [
            0
        ],
        "2": [
            1
        ],
        "3": [
            2
        ]
    }
}

The above will generate 1 http server listening at / at port 8088, and when accessed, the rows matching select * from user limit 10 will be returned.

run

# uses gen/src/resource/graph.json by default, you can specify another json file
# generates the file core/src/bin/demo.rs
php build.php

# run the generated file core/src/bin/demo.rs
cargo run --bin demo