servant-foreign: Expose ordered list of request parts.

[mattjbray]

Allows libraries using servant-foreign (e.g. servant-elm) to generate client functions with argument lists in a consistent order.

See https://github.com/mattjbray/servant-elm/issues/30#issuecomment-347035299 for the motivation.

[alpmestan]

Hello!

Could you add a little bit of haddocks around the new data type, and perhaps even give an indication of how it can be used, maybe with a somewhat minimal example or something? I'm afraid that if we don't do that, we might end up being confused about that ourselves in a year or two =)

[tchoutri]

@mattjbray Hi! Are you still willing to work on this? If so, it can be present on the next major release of Servant. :)

[mattjbray]

@tchoutri apologies for the long silence. Unfortunately I'm not using servant/elm in my current work so I don't think I'll get around to this.

Will close. Feel free to reopen if someone else wants to pick it up.