Overhaul options

[fehrenbach]

Lots of functions take options like limit that do not affect the shape of the result. These should always be exposed.

Other options, like include_docs "change" the result type. Currently we have separate functions for these. It would be good to have a more consistent naming scheme. Or we could do type-level trickery to make the return type dependent on the options (turn Booleans into present/not present). This would probably be bad for inference though, if we don't need the result and don't pass the option, the type would be ambiguous.

Also, mutually exclusive options: key and keys and (startkey or endkey). I think these could be expressed with type-level trickery and no ill effects.