Add a positional decorator

This decorator allow us to define that certain parameters are strictly
keyword arguments only, without sacrificing the convenience of having
those arguments listed in the function definition.

This will mean that we are no longer trapped maintaining backwards
compatibility for the order of keyword arguments.

The enforcement levels should allow us to start putting warnings on
existing functions and strict enforcement on new functions.

Partial-Bug: #1295881
Change-Id: Ic0a196874930e7be8362df50594471ab8037c5b2
Merge "Add a positional decorator"