Simplify loadable extension implementation by providing boilerplate code

[nyurik]

When implementing loadable extension, it seems every/most implementations will have this code (copied mostly from the example). In the spirit of "Rusqlite is an ergonomic wrapper" concept, I think it would be good to provide a macro or some other means to avoid code duplication and complexity in the user's code.

use std::os::raw::{c_char, c_int};
use rusqlite::ffi;
use rusqlite::ffi::SQLITE_NOTICE;
use rusqlite::{to_sqlite_error, Connection, Result};

#[allow(clippy::not_unsafe_ptr_arg_deref)]
#[no_mangle]
pub extern "C" fn sqlite3_extension_init(
    db: *mut ffi::sqlite3,
    pz_err_msg: *mut *mut c_char,
    p_api: *mut ffi::sqlite3_api_routines,
) -> c_int {
    if p_api.is_null() {
        return ffi::SQLITE_ERROR;
    } else if let Err(err) = extension_init(db, p_api) {
        return unsafe { to_sqlite_error(&err, pz_err_msg) };
    }
    ffi::SQLITE_OK
}

fn extension_init(db: *mut ffi::sqlite3, p_api: *mut ffi::sqlite3_api_routines) -> Result<()> {
    let db = unsafe { Connection::extension_init2(db, p_api)? };
    
    // Function registration code goes here

    Ok(())
}