format_username() renamed to user_format_name(), hook_username_alter() to hook_user_format_name_alter()

• format_username() has been renamed to user_format_name() because it is now in user.module.
• Accordingly, hook_username_alter() has been renamed to hook_user_format_name_alter().

Only the names have changed; both the function and the hook are functionally identical to their Drupal 7 versions.

Example:

// Drupal 7
$username = format_username($user);

/**
 * Implements hook_username_alter().
 */
function MYMODULE_username_alter(&$name, $account) {
  // Do whatever.
}

// Drupal 8
$username = user_format_name($user);

/**
 * Implements hook_user_format_name_alter().
 */
function MYMODULE_user_format_name_alter(&$name, $account) {
  // Do exactly the same thing here.
}