Allow user code supplied to iteratees and enumerators to run in its own ExecutionContext. Second part of #807

Also:
- ExecutionContext arguments are now implicit
- Default iteratee ExecutionContexts are public
- ExecutionContexts are prepared before use
Merge pull request #1083 from richdougherty/iteratee-exec-807-part2

Allow user code supplied to iteratees and enumerators to run in an ExecutionContext (#807 part 2)