Fixes to find and eachWithIndex

Fixes:

• An issue where find would cause the process to hang after a certain number of calls.
• @taybenlor: Make sure eachWithIndex passes index then increments (indexing started from 1 instead of 0).

Changes:

• Added +[EKEnumerator new:] convenience initialiser, as an alias for +[EKEnumerator enumeratorWithBlock:].
• Now using instancetype instead of id for all custom initialisers.
• Added EKSemaphore—a wrapper for GCD semaphores. Refactored EKFiber to use EKSemaphore.

My thanks to @taybenlor for the contribution!