impl_table: generated database binding and utils

Example

```rust extern crate chrono;

use chrono::{DateTime, NaiveDate, TimeZone, Utc}; use impltable::{impltable, Table};

// Optionally generate an id column and two timestamp columns: createdat and // updatedat.

[impltable(name = "books", adaptor = rusqlite, withcolumns(id, timestamps))]

[derive(Table)]

struct Book { #[column] pub name: String, #[column] publishedat: NaiveDate, #[column(name = "authorname")] author: String, }

let book = Book { id: 1, name: "The Man in the High Castle".into(), publishedat: NaiveDate::fromymd(1962, 10, 1), author: "Philip K. Dick".into(),

created_at: Utc.ymd(2019, 5, 22).and_hms(8, 0, 0),
updated_at: Utc.ymd(2019, 5, 22).and_hms(8, 0, 0),

}; ```

The above code generates an implementation like the following:

```rust extern crate chrono;

use chrono::{DateTime, NaiveDate, TimeZone, Utc};

struct Book { id: i64, pub name: String, published_at: NaiveDate, author: i64,

created_at: DateTime<Utc>,
updated_at: DateTime<Utc>,

}

impl Book { pub const TABLENAME: &'static str = "books"; pub const ADAPTORNAME: &'static str = "rusqlite";

fn table_name() -> &'static str {
    Self::TABLE_NAME
}

fn all_columns() -> &'static [&'static str] {
    &["id", "name", "published_at", "author_name", "created_at", "updated_at"]
}

fn from_row(row: &rusqlite::Row) -> rusqlite::Result<Self> {
    Ok(Self {
        id: row.get(0)?,
        name: row.get(1)?,
        published_at: row.get(2)?,
        author: row.get(3)?,
        created_at: row.get(4)?,
        updated_at: row.get(5)?,
    })
}

} ```

For more examples see test/sample.rs.