Added lots of unit tests.

Added lots of unit tests.
Switched collections.ReadCollections.BlockToCollectionIndex to collections.ReadCollections.BlockToCollectionIndices since a block can belong to more than one collection.
Made collection.Summarize a method of ReadCollections.
Made a couple testing libraries in blockdigest and collection.
Moved collection.MakeBlockDigest() to blockdigest.MakeTestBlockDigest() (in the testing library).
Created collection.MakeTestReadCollections to simply writing and reading tests (in the testing library).
Created the BlockSet and CollectionIndexSet types to hide some of the awkwardness of using maps as sets and added FromSlice methods.
Moved functions for reading and writing data locally to separate file.
Created separate ReplicationSummaryCounts struct and PrettyPrint method.
Added stats for Collections in addition to Blocks.