[WIP] Error accumulation

[bergmark]

Please file pull requests against this acc-errors branch if you wish to contribute to this feature.

TODOs:

• [ ] Add tests for error accumulation by itself
• [ ] Implement for some built in types / TH / Generics to evaluate API and performance
• [ ] Keep the Simplest example as it was, move the accumulation version to its own example
• [ ] Unexpose liftP2 in favor of <*>+ or consider renaming it.