`OperationId` Combinator

[devinlyons]

Many client generators that work with Swagger use the operationId to name the functions/methods. For servant-swagger, the names are not meaningful and there is no way to override the operationId. I would like to add a combinator to do this. I have already coded it in a library used by my organization and would like to add it to the servant library. It would work much the same way as the Description and Summary combinators.

[devinlyons]

See #1277 for discussion.