Introduced IReflectionElementMaterializer

and AssemblyElementMaterializer, although the implementation is still
incorrect.
Added first implementation of Materialize,

although it's still degenerate.
Passed AssemblyElements through.
Passed any IReflectionElement through.
Preserved ordering during Materialization.
Documented IReflectionElementMaterializer<T>.
Documented AssemblyElementMaterializer.
Deleted the code that passed IReflectionElement

instance through untouched, because I realized that this is a mixed
concern, and furthermore, if all implementations (TypeElementMaterializer,
PropertyInfoMaterializer, etc.) where to do this, a Composite would end up
creating a lot of duplicates.
This is better left to a dedicated component, if required at all.