SecurityDefinitions missing "Ixed" and "At" type-class instances #77

Hi,

The SecurityDefinitions type defined in the OpenAPI library here

Which is a newtype wrapper over Definitions, which in-turn is a type synonym for an InsOrdHashMap Text type.

SecurityDefinitions seems to be missing type-class instances for Ixed and At type-classes, which enables us to use them with Lenses and refer to their indices using at or ix and set a particular SecurityScheme at that index if needed.

The following instances are already present in the Swagger version of the same data-type here.

Opening the issue in addition to #76 here, I'll make a PR soon for this.