This PR upstreams the `ToLevel` typeclass from mathlib and uses it to
fix the existing `ToExpr` instances so that they are truly universe
polymorphic (previously it generated malformed expressions when the
universe level was nonzero). We improve on the mathlib definition of
`ToLevel` to ensure the class always lives in `Type`, irrespective of
the universe parameter.
This implements part one of the plan to upstream a derive handler for
`ToExpr`, as discussed in #5906 and #5909.
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Co-authored-by: Kyle Miller <kmill31415@gmail.com>
Co-authored-by: Tobias Grosser <tobias@grosser.es>
