More Safer Enumerable Methods Please

[markjlorenz]

Things that give back an enumerator need to enumerate over a FetchingSomething
Things that give back a hash or array need give back a FetchingSomething
Things that give back sub-slices (e.g. #last) need to do bounds checking
Remove any Enumerable method that can't reasonably be made to fit these requirements.

For example, #drop should return a fetch'ified array.