.map() .toVector with length constraints

[mio-19]

Is your feature request related to a problem? Please describe.
iron can't detect that methods like .map and .toVector keep length unchanged.

Describe the solution you'd like
Add logic for them on compile-time

Describe alternatives you've considered

[Iltotore]

The problem is unfortunately out of the scope of Iron in its current state. In order to achieve this, the best way would be to redefine map, toVector etc. signatures, maybe in a separated class (look at Cats'NonEmptyList for example).

It might be possible to create a compiler-plugin for that or just support in during compile-time refinement but there are far too many methods in the same situation to handle them all so I'm in favor of not adding specific support for map and toVector and use assume when necessary instead.