rework how canonical and universe mapping works

[nikomatsakis]

As discussed in this Zulip thread, the existing canonical logic is broken:

• First, map_from_canonical needs to remap the universes in the canonical binders
• Second, we need to preserve all universes from the input, not just those that appear in placeholders

Finally, this might be an opportunity to refactor canonical so that it canoncializes both inference variables and placeholders, like rustc does. This would permit us to remap univeses by just adjusting the canonical meta-data.

Steps to fix:

• Create some test cases that show how current behavior is wrong
• Fix current behavior first, perhaps?
• Then do the nicer refactoring?

[nikomatsakis]

@rustbot assign @Areredify