narrow types: Make Narrow an opaque type.

In the preceding series of commits, we've taken all the places where
our code used to depend on the details of our internal representation
of narrows, and converted them so they use reasonable explicit
interfaces provided by the `narrow` module.

This commit recruits Flow to confirm for us that that job is complete.
Because `Narrow` is now an opaque type, code outside this module now
has to treat it as if it could mean anything: so it can't create a
value of this type any more than it could of type `empty`, and can't
consume one or break it down any more than it could a value of type
`mixed`, the supertype of all types.

Effectively this means that code outside `narrow` can only create a
`Narrow` by invoking something inside the module; can then pass it
around and store it however it pleases; but then can only inspect the
information inside it by passing it back to something within the
module, or to an operation like `===` or `JSON.stringify` that's
completely generic so that it actually accepts type `mixed` as input.

(The way this series was developed is that this commit came first,
immediately after distinguishing the `ApiNarrow` type and making
`Narrow` an alias of it in the first place.  That produced a long
list of Flow errors, which served as a to-do list to make the other
changes in the series.  Once complete, this commit was rebased to the
end so that, as always, no commit in the series breaks our tests.)

src/utils/narrow.js

@@ -31,7 +31,7 @@ import {
  *  * `ApiNarrow` for the form we put a narrow in when talking to the
  *    server, and `apiNarrowOfNarrow` for converting to it.
  */
-export type Narrow = ApiNarrow;
+export opaque type Narrow = ApiNarrow;
 
 export const isSameNarrow = (narrow1: Narrow, narrow2: Narrow): boolean =>
   Array.isArray(narrow1) && Array.isArray(narrow2) && isEqual(narrow1, narrow2);