std: Improve vec::ChunkIter

Implement clone, bidirectionality and random access for this iterator
std: Add iterator::Repeat to repeat an element endlessly
std: Rewrite the HashSet set operation iterators

Use the Repeat iterator to carry the "explicit closure capture" that was
previously done with the custom EnvFilterIterator.
std: Use method name Option::consume

With Option as the simplest container, `consume` is the way to turn it
into a by-value iterator.
std: Improve the documentation for iterator::Invert