Add `asserts` annotations

[mmkal]

Fixes #159

Also adds:

• Type-level tests
• A generic typearg for ArrayPredicate and ObjectPredicate. So ow.array.ofType(ow.string) infers string[] and ow.object.exactShape({ foo: ow.string, bar: ow.object.exactShape({ baz: ow.number }) }) infers { foo: string; bar: { baz: number } }
• A Shape -> TypeOfShape helper

Changes:

• The optional modifier now makes each predicate ThePredicate<T | undefined> rather than Predicate (which could lead to uncaught NREs)
• The hidden create-constructor-function-thing is no longer allowed to receive undefined for any value, since undefineds should now be properly tracked.

[sindresorhus]

Thanks for working on this 🙌

[sindresorhus]

This looks great! Thank you :)