Refactor http messages to be class-like

[EricWF]

Currently HTTP messages are POD's. There are many external functions that make more sense as methods.

Currently I think direct access to data should remain possible. This may change.

[daniel-dressler]

HTTP is used for binary formats like IPP so direct data access may be needed long-term. Instead would making it const be sufficient?

[EricWF]

I think you'll always have raw access to the data after the header. The struct currently has a collection of fields as well (Key-Value pairs). I was thinking of guarding access to the list of fields.

[daniel-dressler]

Good idea then, guarding access to the raw list makes sense.