Optimize ModuleHandler::invokeAll()

Looking at ModuleHandler::invokeAll() (or module_invoke_all() in D7), I see a number of possible optimizations.

1. The recursive result merging is not necessary in many cases. This can be discussed separately, see 📌 Provide different versions of ModuleHandler::invokeAll() for performance Active
2. Replace $function = $module . '_' . $hook with $function = $module . $suffix, where $suffix can be built outside the loop.
3. The result of this string concatenation can be cached in a member variable.
4. call_user_func_array() can be replaced with $function($x, $y, $z), at the cost of making it more verbose and less DRY. Yes, I am suggesting to copy the main part of the function for different parameter counts.
5. Replace isset($result) with NULL !== $result.
6. Provide a version of NestedArray::mergeDeep() which accepts exactly two parameters and uses its own implementation instead of calling self::mergeDeepArray(). There is unnecessary indirection and parameter juggling.

I played with changes equivalent to these on a Drupal 7 site, where module_invoke_all('permission') got measurably faster.
Real benchmarks have yet to happen.