Avoid using Object.keys(obj).length in hot code paths

Complexity of Object.keys(obj).length is O(n) where n is the number of
object keys. Unfortunately, JavaScript have no built-in means of
determining number of object keys in constant time. Therefore, we have
to count object keys using separate counter variable.

It may look ugly (and indeed it is), but it greatly improves smoothness
and overall feel of IITC.

Related: #409