reverse iterators for Julia 0.7

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

Inspect the IterTools iterator types and compare their reverse-iteration behavior with Julia's base/iterators.jl. Start with cases where Iterators.reverse can delegate to an inner iterator, then identify types needing specialized Reverse iteration methods. Done means the applicable IterTools iterators support reverse iteration under Julia 0.7's protocol.