Introducing ByteCounter

ByteCounter will replace big.Int as data structure for ordering of
events. It is more dense in information.
Use ByteCounter EvIds and grouping based on stream

 * ...thus, big.Int dependency is now gone.
 * Currently missing is the initialization code for streamIdGenerator.
   It needs to iterate through the streams and extract their
   corresponding ID:s.
Making `EventStreamComparer` "private"

...in the sense that it is not reachable outside of its package.
Documentation of functions and methods

No logic modified.
Changing []byte => StreamName

...where appropriate.
Changing []byte => EventId

...where appropriate.
Making `ByteCounter` private

...in the sense that it is not reachable outside of its package.