[TASK] [CODE QUALITY] Design simple declarative syntax for workitem validation

[iamer]

Timeboxed to 10 hours

Design a way for participants to declare which fields and parameters they expect in their workitems, as well as constraints on their values (such as type, element type, string-matches-regexp, string-in-enum, that kind of thing).

Design considerations:

• The declarations should be in the participants, so that people will
  actually update them as the participant changes
• The Exo should guarantee that the constraints are met before the
  participant gets the workitem.
• The declarations should be passive data structures, so that they
  can be processed without running participant-specific code.
• The syntax should support concepts like "integer as string" and
  "list as string" because ruote often passes stringified parameters
  like that.

Possibilities:

• Declaring optional parameters may be useful for documentation,
  and allows typechecking on those parameters
• Declaring output fields may be helpful in the future for
  automated checking of compatibility between participants
• It may be possible to generate documentation from these declarations,
  but don't let that complicate the basic design.
• Alternately, it may be possible parse the declarations from the docstring.

The output of this task should be a document describing the
syntax and suggested implementation, ready for discussion.

[iamer]

From pami :

http://www.alexconrad.org/2012/01/make-validation-suck-less.html

And the validictory JSON validation library seems quite promising

https://github.com/sunlightlabs/validictory