VectorConstruction

A "mini-DSL" that describes construction of a vector. Vector can be constructed from various range operations (relocate, drop, slicing, appending), by combination of two vectors or by taking a vector from a list of variables.

Notably, vectors can only be constructed from other vectors of the same type (the Combine operation requires this - even though that one could be made more general). This is an intentional choice to make the representation simpler.

Logically, when we apply some index operation, we should get back a polymorphic vector construction (\forall T. VectorConstruction<T>) that can be applied to variuous different vector types. That would mean adding some more types, so we just model vector construction as an untyped operation and the typing is resquired by the Build method of the vector builder.