[INTERNAL] sap.ui.base.DataType: Simplify interface implementation
[INTERNAL] sap.ui.base.DataType: Simplify interface implementation

- Using a Set instead of an object-map
- Fixing ESLint error by not using a string object, which is not required
  (also see inline comment about the 'ObjectPath.set' call)

Change-Id: Ia788c4313ce416772cdd5f8552b8293e7e5fbdb4
JIRA: CPOUI5FOUNDATION-379

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5