Implement merge and mergeAll (dart-lang/stream_transform#7)

Closes dart-lang/stream_transform#4 

merge and mergeAll combine/interleave streams. Handles both broadcast
and single-subscription streams.

Add implementation of containsAllInOrder in test/util - once this is
submitted in package:matcher it can be removed here. Submitting here
since the other package is blocked.

Remove -dev from pubspec for publishing.