Monoid instances are wrong

[fisx]

for instance:

• the one in _swaggerPaths should treat the values as monoids and merge them.
• PathItem should merge, but overwrites.
• the on in _operationResponses (inside the Responses type) should overwrite. oh, and actually does. good.

proposed solution:

wrap InsOrdHashMap types in newtypes and give them individual instances. to be more specific, add newtype Merging a = Merging a and newtype Overwriting a = Overwriting a, and give instances for some concrete a as needed.

[fisx]

I'm also not sure about SwaggerMonoid, which differns in very few places, and it's hard to see which those are. Perhaps we can get rid of this somehow as well?

[Taneb]

It looks like the issue with _swaggerPaths has been fixed on GitHub but not yet released (although I haven't tested it). The problem with how operations are merged still stands, though.