Create is_countable() to check if it's safe to call count()

[duncan3dc]

Following the recently accepted rfc (and pr #2185) it's now a little awkward to avoid warnings when calling count():

is_array($thing) || $thing implements \Countable

It's also not possible to check if an object has a count_elements handler and can be safely passed to count().

This pr adds a new function to address both of the above issues:

is_countable($thing);

[php-pulls]

Comment on behalf of krakjoe at php.net:

labelling