[phobos] advance() for infinite random access ranges?

[David Simcha]

I've thought some more about how to support slicing in the case of 
take(someInfiniteRange, someNumber) and I think it would be a good idea 
to require infinite random-access ranges to support an advance() 
function.  This would basically be like slicing, but only change the 
beginning of the range, not the end (since there is no end of an 
infinite range).  Calling infiniteRange.advance(N) would be equivalent 
to calling infiniteRange.popFront() N times, except that advance() would 
have O(1) time complexity.  Sound good?  Any alternative suggestions?