Fix "for...of" loop on Result<T> objects

[ramikg]

Trying to iterate over a Result<T> object (e.g. one returned by calling find) using a "for...of" loop results in the following TypeScript error:

TS2488: Type Result must have a [Symbol.iterator]() method that returns an iterator.

But, Result<T> explicitly implements [Symbol.iterator], so ignoring this type error fixes the "for...of" iteration.

This PR changes Result<T> from being an Iterator to being an IterableIterator (which means that it implements [Symbol.iterator]() in addition to being an Iterator).

[ramikg]

This PR originally added the method [Symbol.iterator] to the type definition of Result<T>.

The correct solution is to treat Result<T> as an IterableIterator.