reverse iterators for Julia 0.7

[stevengj]

Now that 0.7 has reverse iteration (https://github.com/JuliaLang/julia/pull/24187), it would be nice to support that in the IterTools iterators where it makes sense:

• define Iterators.reverse(itr::SomeIterType) in cases where the reverse iterator is easily written in terms of some other iterator type. (e.g. if itr "wraps" around another iterator, just call Iterators.reverse on the "inner" iterator).

• define start/next/done methods for Iterators.Reverse{SomeIterType} in cases where you need a specialized iteration protocol.

For examples, see https://github.com/JuliaLang/julia/blob/master/base/iterators.jl