Add last_insert_rowid method to SqliteConnection and RawConnection

[Alexi24601]

Exposes SQLite's sqlite3_last_insert_rowid() through SqliteConnection, following the same pattern as the existing rows_affected_by_last_query.

Returns Option<i64> — None if no INSERT has been performed on the connection, Some(rowid) otherwise.

Example

use diesel::prelude::*;
use diesel::connection::SimpleConnection;
use diesel::sqlite::SqliteConnection;

let conn = &mut SqliteConnection::establish(":memory:").unwrap();
conn.batch_execute("CREATE TABLE users (id INTEGER PRIMARY KEY, name TEXT NOT NULL)").unwrap();

assert_eq!(conn.last_insert_rowid(), None);

conn.batch_execute("INSERT INTO users (name) VALUES ('Sean')").unwrap();
assert_eq!(conn.last_insert_rowid(), Some(1));

conn.batch_execute("INSERT INTO users (name) VALUES ('Tess')").unwrap();
assert_eq!(conn.last_insert_rowid(), Some(2));

Notes

• Only affected by successful INSERTs into rowid tables
• Not changed by failed INSERTs, UPDATEs, or DELETEs
• Scoped per-connection

@LucaCappelletti94 made me aware of the need to do this, so i tried my hand at this. It is still among the very first pr i do, i am still learning Rust and how to properly use git.

[LucaCappelletti94]

@weiznich LGTM, the only thing that could be improved at all could maybe be to replace the INSERT/DELETE raw SQL in the test submodules with actual ORM calls if I really were to be extra nitpicky