Variable chunk sizes operations

[Mihaela Chirea]

I started working on this issue and I reached a point where some 
more opinions would be needed:

https://github.com/dlang/phobos/pull/7600

Since the range is split into uneven parts, some operations can 
only be done in O(n).
Slicing requires taking into account all the first elements of 
the range specifying the chunk lengths to find the starting point 
in the source range.
Back and popBack also need to find where that last chunk is since 
the source length and the provided lengths wouldn't necessarily 
match.

As mentioned in the pull request comments, the complexity of the 
program using these might be higher than expected.

So should they be provided in spite of their complexity? Or would 
a forward range(at most) be enough?