FallibleIterator is missing many methods from the modern std::Iterator

[jimblandy]

Since FallibleIterator was created, std::Iterator has continued to evolve and grow interesting features. It would FallibleIterator easier to use if people could assume it has all the features they're used to, just in a failure-friendly form.

Missing methods, as of a skim through the docs today:

• step_by
• for_each
• skip_while
• take_while
• skip
• scan
• flat_map
• flatten
• inspect
• partition
• find_map
• max_by
• min_by
• unzip
• cycle
• sum
• product

These probably aren't necessary, since they're just providing what FallibleIterator is designed to provide anyway.

• try_fold
• try_for_each

[sfackler]

Yep, we should mirror what the standard library offers.