Add copied, sum, and product methods

[aprilwade]

This adds the Iterator methods that FailableIterator is currently missing, resolving #15. I didn't write tests for sum or product since they just use Result's Sum and Product impls. I also didn't provide a test for copied since cloned has no tests. Naturally, if you think these need tests, I'll write them.

[sfackler]

The use of Option::copied increases the minimum supported Rust version, so it's probably best to avoid calling that method and just do the deref manually: https://circleci.com/gh/sfackler/rust-fallible-iterator/104?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-checks-link