fix: remove optional status from KeysByType keys

Because status modifiers are attached to keys, `KeysByType` gives back
an `undefined` key when the original object contains optional keys; even
if the optional key is not of the given type. This change fixes that by
removing the optional status modifier from the object while iterating
over the keys.

The downside to this change is that it does not perfectly purify the
object. So if other status modifiers are added in the future, we will
not necessarily be robust to that change. A more "pure" fix (pun
intended) would be to work with a purified object. The downside is that
the only way I could figure out to do that would require much uglier
code and/or a helper function that would need to be exported. In favor
of minimizing "private" exports, I opted for the simple solution.

Also a downside here is that we cannot filter by `undefined` if a key
is optional. This was already an issue before this commit, so I'm
punting on it for now because the fix seems non-trivial.

Closes #106

src/types/objects.ts

@@ -219,7 +219,7 @@ export type DeepReadonly<T> = Readonly<{
  * @returns keys of `O` whose right-side value is `T`
  */
 export type KeysByType<O extends object, T> = {
-    [k in keyof O]: O[k] extends T ? k : never;
+    [k in keyof O]-?: O[k] extends T ? k : never;
 }[keyof O];