Solve performance of values iterators

[udoprog]

This PR solves all iterator performance issues by ensuring that the used iterator implementation that is produced by types such as Set and Map all directly relate to an iterator adapter provided by core::iter (i.e. std::iter).

After some investigation it was discovered that the difference in performance was due to different code generation, in particular when using an appropriately sized array directly it will be unrolled by rustc and LLVM https://godbolt.org/z/aW1fqa4xM.

This is very likely due to the internal use of TrustedLen, which is an unsafe trait the the compiler can use to determine the exact length of an iterator https://doc.rust-lang.org/src/core/iter/adapters/flatten.rs.html#118.

So by using internal iterators even though it leads to more complex typing (somewhat mitigated through aliases) we're able to convince rustc to peel of all abstractions and as appropriate make use of loop unrolling. This means that for example the Map::values iterator should behave exactly like an [Option<T>; SIZE] iterator for non-composite keys.