Reimplement `SrcField` and `SrcVector` as Protocols

lang.SrcField and lang.SrcVector should be dropped and instead be replaced by appropriate protocol classes (e.g. SupportsFieldExtraction and SupportsVectorExtraction). This would remove the restriction that objects implementing these protocols have to be AugExprs. The IFieldExtraction ABC can be removed in the process, as its interface can become the interface of SupportsFieldExtraction.