Add some more sequence functions

While building a react app I found myself wanting `partition-all` and `take-nth`, so I've added them here.

I initially ported the implementations into my app, but ended up implementing as eager recursion as `lazy-seq` was not available - so here it is too.

I'll update the docs too soon - how would you feel about putting the docs in master with a script to update gh-pages? I think this would make it easier to keep them in sync.