Implement to_a as a companion to first and last on reader API

[mostlyobvious]

It is not unfrequent to see event_store.read.stream('some_stream').each.to_a whenever you want fetch events besides iterating on them.

We've added first and last already for convenience, this seems like a good addition too. We've debated whether to include whole Enumerable before. I doubt now we need all of Enumerable and that is achievable via each returning Enumerator anyway.