Prevent duplicate IDs from being used in OAS 3 parser

[kylef]

The OAS 3 parser has an internal context which includes a method registerId() which can be used to register IDs which are placed in the API Elements parse result. This function is used to know if the IDs are unique. There are a couple of problems with the current implementation:

• registerId is not used everywhere
• we should implement erroring out when duplicate ID is used, it may be possible to write as OAS 3 document which contains the multiple elements with the same ID (security definition and schema sharing same name).
• Certain IDs are unfortunately not possible due to how API Elements works and we must either escape or prevent using known element names as an ID. For example if the ID was httpRequest, string, resource, category etc, this would clash with the already existing element names.

This problem likely applies to API Blueprint and the OAS 2 parser too.