Make Descriptor creation more type-safe

[mvriel]

In the AssemblerInterface's create method (and thus every assembler underneath) we are unable to apply proper type-hinting. This opens the door for various type-related bugs.

To decrease the chances of these type of bugs creeping in we want to change the Descriptor creation process in such a way that a real type-hint can be used in the create function.

This is more effort than just using a type-hint; we may need to change the interfaces of some classes or even introduce a new interface or abstract class to help with this.