Allow assign to take a map

[amclain]

Description

This PR allows assign to take a map of values, like Phoenix does.

Motivation and Context

When working on a Scenic project, we captured the entire scene.assigns and transformed a few values. We received an error when trying to assign this resulting map back to the scene, since scene.assigns gave us a map. We referenced Phoenix's behavior of assigns since that was motivation for bringing it into Scenic, and Phoenix does in fact allow both a keyword list or map as assigns. Therefore we thought we should allow for assigning a map in Scenic as well.

Happy to discuss if this either isn't desired or should be implemented differently.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature
  but make things better)

Checklist

• Check other PRs and make sure that the changes are not done yet.
• The PR title is no longer than 64 characters.