[suggestion] implement std::iter::DoubleEndedIterator

[petar-dambovaliev]

Hello, i have a use case for parsing backwards.
I am writing a library to calculate tax returns from stock sales and some brokers export large csv files containing all transactions for a given period.
The rows are ordered as first at the top and last at the bottom.
If i can use DoubleEndedIterator::next_back, i can prevent reading the whole file in memory, to construct a VecDeque.