Move back to 2.10
Bug fixes, improvements, and new examples.

1. The start parameter is now a thunk, which means if the user wants to
initialize something mutable (i.e. ArrayBuffer) they can do it safely.

2. The nth() method is now 1-indexed (if we want the 100th item we will pass
100). This might be controversial but for now I found it easier than having to
remember that to get the 1000th factorial I needed to pass 999.

3. Added another version of the rational stream that uses Spire.

4. Added a naive prime generator. On my computer I can get the millionth prime
number (15485863) in ~7s, so it's not doing too badly.