Teach provablyDisjoint to fold compile-time constants

[dwijnand]

No description provided.

[johnynek]

@dwijnand thanks for working on this!

Would you be willing to see if my example works now, and if so add it as a new test case:

#16596 (comment)

[johnynek]

also, could we add tests with Count[4, Int], Count[5, Int] etc... just to be sure (it surprised me before that small examples worked but bigger didn't).

[dwijnand]

Your summon[Nat.FromInt[3]] was still failing with this change (as well as Count1[3, Int] and greater failing in -Ytest-pickler). So I bit the bullet and finally decided for #18073.