fix slices

[Mike Bayer]

we've had some reports of slices like 0:0 and such not working, and even a slice like [:1000](:1000) is apparently not working (needs [0:1000](0:1000)). Fix up slices to be less error prone.

[Mike Bayer]

in 5f75197e86059d7e4bbe20558c98011d89f9cfda, we calculate LIMIT/OFFSET for all slices except those which are known to slice from the end of the list. Slices with no length don't execute a query.

[Mike Bayer]

Removing milestone: 0.5.0 (automated comment)