New algo

Closes #13 (closed), #62 (closed), #91 (closed), #26 (closed)

Plan:

• Check that the algorithm works
• Ensure that implicit works with the new algo, might require modifying auxiliary series
• Replace general with the new algo
• Update tests
• Remove expanded and symbolic, as well as the logic for selecting these
• Update docs
• Add to changelog. Since this is a big change, decide to which version this belongs
• Cleanup the API. For example, the new algorithm doesn't use exclude_last.