Convert OpenAPIv2 concepts into their equivalent OpenAPIv3

[karlvr]

And to this in the core so that our model only includes OpenAPIv3 concepts.

• Remove File type and convert to string+binary. The File type is a left-over from OpenAPI v2 and I think we should convert it to string+binary and use our support for that.
• form parameters can be converted to an application/x-www-form-urlencoded body
• remove collectionFormat from CodegenParameterBase and convert to encoding styles