This PR adds level instantiation and normalization in `getDecLevel` and
`getDecLevel?` before calling `decLevel`.
`getLevel` can return levels with uninstantiated metavariables or
un-normalized structure, such as `max ?u ?v` where the metavariables
have already been assigned. After instantiation and normalization (via
`normalizeLevel`), a level like `max ?u ?v` (with `?u := 1, ?v := 0`)
simplifies to `1 = succ 0`, which `decLevel` can decrement. Without this
step, `decLevel` sees `max ?u ?v`, tries to decrement both arms, fails
on a zero-valued arm, and reports "invalid universe level".
Concretely, this fixes `for` loops with `mut` variables of
sort-polymorphic type (e.g. `PProd Nat True`) where the state tuple's
universe level ends up as an uninstantiated `max`.
The expected-output change in `doNotation1.lean` is because the `for`
loop's unit type now resolves to `Unit` instead of `PUnit` due to the
improved level handling.
