Refactoring sum functions

- All sum functions, including `sum(x)`, `sum(f, x)`, `sumabs(x)`, and `sumabs2(x)` now use the same core implementation, namely `sum_seq` and `sum_pairwise`. Everything else is just light-weight wrappers.

- I did some simple benchmarks myself, which shows that this implementation will not compromise runtime efficiency.

- `sumabs` will be used to implement `vecnorm1`, and `sumabs2` will be useful for implementing `var`.