Feature/language coverage

This is a work around a clang-10 bug

Add associate tests

On the way, fixed a few minor issues:
- simplified conversion to string
- fixed missing alternative when converting to tuples
- change a few copies into moves

Replace old values (when leaving a block) by default (constructor)
values. This was buggy since old values were replaced by a
`std::monostate` which led to inconsistent state of local variables.

Actually, for instance, list of tuples of R^2 defined as
`((1,2), (2,3))`
were treated correctly but in the case of a single element
`((1,2))` was interpreted as `(1,2)` which is a tuple of natural
integers.