Implement fittings nodes.

[sparkpunkd]

Complete the basic implementation of the fittings nodes. Future enhancements are tracked as a separate issue.

• [ ] take
• [ ] doto
• [ ] map
• [ ] filter
• [ ] head
• [ ] last
• [ ] throttle
• [ ] rate-limit
• [ ] retime
• [ ] concat
• [ ] merge
• [ ] otherwise