Add bindings to automatic extension loading API (#1487)

* Add bindings to automatic extension loading API

it doesn't seem possible to directly register an `AutoExtension`.

tests/auto_ext.rs

@@ -0,0 +1,41 @@
+#[cfg(all(feature = "bundled", not(feature = "loadable_extension")))]
+#[test]
+fn auto_ext() -> rusqlite::Result<()> {
+    use rusqlite::auto_extension::*;
+    use rusqlite::{ffi, Connection, Error, Result};
+    use std::os::raw::{c_char, c_int};
+
+    fn test_ok(_: Connection) -> Result<()> {
+        Ok(())
+    }
+    unsafe extern "C" fn sqlite_test_ok(
+        db: *mut ffi::sqlite3,
+        pz_err_msg: *mut *mut c_char,
+        _: *const ffi::sqlite3_api_routines,
+    ) -> c_int {
+        init_auto_extension(db, pz_err_msg, test_ok)
+    }
+    fn test_err(_: Connection) -> Result<()> {
+        Err(Error::SqliteFailure(
+            ffi::Error::new(ffi::SQLITE_CORRUPT),
+            Some("AutoExtErr".to_owned()),
+        ))
+    }
+    unsafe extern "C" fn sqlite_test_err(
+        db: *mut ffi::sqlite3,
+        pz_err_msg: *mut *mut c_char,
+        _: *const ffi::sqlite3_api_routines,
+    ) -> c_int {
+        init_auto_extension(db, pz_err_msg, test_err)
+    }
+
+    //assert!(!cancel_auto_extension(sqlite_test_ok));
+    unsafe { register_auto_extension(sqlite_test_ok)? };
+    Connection::open_in_memory()?;
+    assert!(cancel_auto_extension(sqlite_test_ok));
+    assert!(!cancel_auto_extension(sqlite_test_ok));
+    unsafe { register_auto_extension(sqlite_test_err)? };
+    Connection::open_in_memory().unwrap_err();
+    reset_auto_extension();
+    Ok(())
+}