Flatten and FlatMap with constant inner iterator size

This Rust package provides ConstSizeFlatten and ConstSizeFlatMap which make use of the also provided ConstSizeIntoIterator to know how many items they will yield.

Note that core & std already provide this functionality for some types through a hack using specialization. This crate’s contribution is that the trait ConstSizeIntoIterator is public and the functionality is therefore extensible.