ledb-derive 0.4.0

# Derive macro for defining storable documents

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This **derive macro** helps to define documents which can be managed using persistent storages like *LEDB*.

The **LEDB** is an attempt to implement simple but efficient, lightweight but powerful document storage.

The abbreviation *LEDB* may be treated as an Lightweight Embedded DB, also Low End DB, also Literium Engine DB, also LitE DB, and so on.

## Links

* [ledb-types Crate on crates.io](https://crates.io/crates/ledb-types)
* [ledb-types API Docs on docs.rs](https://docs.rs/ledb-types)
* [ledb-derive Crate on crates.io](https://crates.io/crates/ledb-derive)
* [ledb-derive API Docs on docs.rs](https://docs.rs/ledb-derive)
* [ledb Crate on crates.io](https://crates.io/crates/ledb)
* [ledb API Docs on docs.rs](https://docs.rs/ledb)

## Usage example

```rust
use serde::{Serialize, Deserialize};
use ledb::{Document};

#[derive(Serialize, Deserialize, Document)]
struct MyDoc {
    // define optional primary key field
    #[document(primary)]
    id: Option<Primary>,
    // define unique key field
    #[document(unique)]
    title: String,
    // define index fields
    #[document(index)]
    tag: Vec<String>,
    #[document(unique)]
    timestamp: u32,
    // define nested document
    #[document(nested)]
    meta: MetaData,
}

#[derive(Serialize, Deserialize, Document)]
#[document(nested)]
struct MetaData {
    // define index field
    #[document(index)]
    keywords: Vec<String>,
    // define other fields
    description: String,
}
```

This automatically generate `Document` traits like so:

```rust
impl Document for MyDoc {
    // declare primary key field name
    fn primary_field() -> Identifier {
        "id".into()
    }

    // declare other key fields for index
    fn key_fields() -> KeyFields {
        KeyFields::new()
            // add key fields of document
            .with_field(("title", String::key_type(), IndexKind::Unique))
            .with_field(("tag", String::key_type(), IndexKind::Index))
            .with_field(("timestamp", u32::key_type(), IndexKind::Unique))
            // add key fields from nested document
            .with_fields(MetaData::key_fields().with_parent("meta"))
    }
}

impl Document for MetaData {
    // declare key fields for index
    fn key_fields() -> KeyFields {
        KeyFields::new()
            // add key fields of document
            .with_field(("keywords", KeyType::String, IndexKind::Index))
    }
}
```