Add skip_serializing_if_default field attribute

[Gordon01]

This attribute skips field serialization if its value equals default.

Motivation:

Imagine we have an application, where the config may be changed dynamically and needs to be saved. Or just be saved for some reason.

#[derive(Serialize, Deserialize, Debug)]
struct Config {
    #[serde(default = "default_version")]
    version: u32,
    #[serde(default = "default_author")]
    author: String,
}

Probably we have a lot of fields that just defaulted and saving them will add unnecessary noise to the config file.

We may use skip_serializing_if attribute and provide functions that check if the value is default:

#[derive(Serialize, Deserialize, Debug)]
struct Config {
   #[serde(default = "default_version", skip_serializing_if = "is_version_default")]
   version: u32,
   #[serde(default = "default_author", skip_serializing_if = "is_author_default")]
   author: String,
}

fn is_version_default(value: &u32) -> bool {
   *value == default_version()
}

fn is_author_default(value: &String) -> bool {
   *value == default_author()
}

This adds lots of one-line functions and noise to the code.

Solution:

Use skip_serializing_if_default attribute from this PR:

#[derive(Serialize, Deserialize, Debug)]
struct Config {
    #[serde(default = "default_version", skip_serializing_if_default)]
    version: u32,
    #[serde(default = "default_author", skip_serializing_if_default)]
    author: String,
}

It calls the field's default function, compares its current value, and if it matches with the value provided by the default function, skips serialization.

[Gordon01]

I've added a test for this feature and fixed the wrong struct length that was revealed by the test. I don't like the code duplication, but it seems to me it's the style you prefer.

Any advice on how it should be?

Otherwise, the feature seems to be complete.