Mapping and reducing over heterogenic tuples

[mangelats]

Hi there!

I'm working in another library and I need to map and reduce an heterogenic tuple. I've come to a working implementation and I was wondering if you would be interested in me adding it to this project. Here is proof of concept in the playground. Note that this have no macros yet and it's manually created up to 3 elements.

The main akward part of the solution is that you cannot pass a generic function as argument, so I used a ZST with a trait implementation.