Add a Prim.Row.Closed compiler solved class #3800

I've found the occasional want to do RowToList magic on a function that takes a record as an argument. The problem is these are inferred polymorphically, or at least the record tail is left unknown.

conjure ::
  forall r rl z.
  RowToList r rl =>
  Conjure rl r =>
  ({ | r } -> z) -> z
conjure = ?conjure

example =
  conjure \{ foo, bar } -> ...

Something like this doesn't compile because the type of r in this case will involve an unknown for the tail and RowToList requires an empty row terminator. Making this compile requires a type signature on the lambda closing the row. However, if we can conjure a record with only the required fields, this is totally safe.

Could we have something like

class Closed :: forall k. Row k -> Row k -> Constraint
class Closed open closed | open -> closed

Where the compiler yields a closed row with only the known components of the row? This would allow us to type the example as

conjure ::
  forall r rl z.
  Closed r r =>
  RowToList r rl =>
  Conjure rl r =>
  ({ | r } -> z) -> z

Forcing the unknown tail to unify with ().