Create assignment order proposal

[josh11b]

trafficstars

Description of the bug:

Question-for-leads issue #686 has a decision for how assignment works for classes by default. This decision should be implemented in a proposal (as noted at the end of the issue).

Link to documentation:

https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/classes.md#assignment

[rajeck1234]

@josh11b can i try this one

[josh11b]

@josh11b can i try this one

@rajeck1234 : See: https://github.com/carbon-language/carbon-lang/blob/trunk/CONTRIBUTING.md#implement-carbons-design , we don't assign issues to new contributors, but that should not stop you from fixing this issue.