feat: add jmap to simplify json unmarshal method

[sleagon]

Add a simpler method to solve precision losing problem in json unmarshal method.

Here is simple example:

    val := `{"a": 9223372036854775807, "b": 1.2e8}`
    data := make(map[string]interface{})
    err := json.Unmarshal([]byte(val), &data))

   log.Println(data["a]) // => 9223372036854776000 (float64)

Just replace one line code as follows:

original:

    err := json.Unmarshal([]byte(val), &data))
    // => 9223372036854776000 (float64)

to:

    err := json.Unmarshal([]byte(val), Ptr(JMap(data)))
    // => 9223372036854775807 (int64)

whole code as follows:

    val := `{"a": 9223372036854775807, "b": 1.2e8}`
    data := make(map[string]interface{})
    err := json.Unmarshal([]byte(val), Ptr(JMap(data)))

   log.Println(data["a]) // => 9223372036854776000 (float64)