`join` function is missing, to join a few bindings

[yegor256]

Would be great to have an ability to join two or more bindings:

name: foo
pattern: ⟦ 𝐵1, x ↦ "foo", 𝐵2, y ↦ "bar", 𝐵3 ⟧
result: ⟦ 𝐵4 ⟧
where:
  - meta: 𝐵4
    function: join
    args: ['𝐵1', '𝐵2', '𝐵3']

The join function must rename all attributes, so that all of them in 𝐵4 are unique.

[yegor256]

@maxonfjvipon also, we should think about \rho. It may exist in every 𝐵, but should only exist once in the resulting binding.