Fast row values pagination

[nitely]

Fixes #9
Related #8

Implements the "row values" variation.

Benchmark without index, 1M rows; 4 sort fields; page 80K; article 800K:

Offset/Limit 1.692431215000397
Seek Method 0.3435570509991521

Benchmark with index, 1M rows; 4 sort fields; page 80K; article 800K:

Offset/Limit 0.10353935199964326
Seek Method 0.0020750300009240163

[nitely]

This cannot be merged as is. The "row variation" doesn't support mixing the sort direction, all fields must be either ASC or DESC. This implementation does not support relationships (ex: foo__bar) on the filter/sort fields. It seems to work on sqlite and postgresql, but it won't work on Oracle, and idk about mysql. It may not work with every field; I've tested it with BooleanField, DatetimeField, and IntegerField.

I think there should be a parameter that enables this, like fast_mode=True or something.