Consider making order meaningful in "oneOf" combo scheme

[mmccool]

The "oneOf" combo scheme lists sub-schemes in an array. This issue proposes that we make the order meaningful by adding an assertion to the TD spec that consumers SHOULD implement schemes earlier in the list if possible.

[mmccool]

During the TPAC meeting @relu91 mentioned "formIndex" and maybe we should add "securityIndex". The "formIndex" is used in scripting to access/select the order of forms. However, if the runtime of the consumer implementation "automatically" does security, it might be better to just have that implementation always try to access security in order, starting with the first. There may be no reason to expose this at the scripting API level.