Key-As-Segment for multi-part keys

Generalize the rules in ODATA-799 to allow key-as-segment notation also for multi-part keys.

Example: an accounting document is identified by the company code, the year, and the (auto-incremented) document number:

<EntityType Name="AccountingDocument">
<Key>
<PropertyRef Name="CompanyCode" />
<PropertyRef Name="Year" />
<PropertyRef Name="DocumentNumber" />
</Key>
</EntityType>

Taking into account the order of <PropertyRef> elements within the <Key> element a single accounting document can be identified as

GET AccountingDocuments/SAP/2016/4329043280

Open a new issue (ODATA-976) to track behavior of partial key specification. In the meantime, specification of partial keys is undefined, and if one segment is resolved as a partial key value then the subsequent segments should be resolved as any remaining key values.