…. of'

When using the docs TypeScript declaration file, the example async iterator
code...

    const dirIter = await tjs.readdir('.');
    for await (const item of dirIter) {
        console.log(item.name);
    }

...does not compile due to a TypeScript error.

    error TS2504: Type 'DirHandle' must have a '[Symbol.asyncIterator]()' method that returns an async iterator.

The DirHandle must be declared as an AsyncIterableIterator, which the
implementation already satisfies, instead of an AsyncIterator.

The MDN reference on `for await ... of` hints at the need for this:

    Note: for await...of doesn't work with async iterators that are not async iterables.

The UTF-8-only encoder/decoder was performing poorly due to inefficient
array manipulation.
Applied changes from the original text-encoding package that reduce
the amount of copying.

Relates to #316

Including the win32 implementation.

Checks correctness on Windows.

In the normalizer, `base_name` is the full path to the module doing the
import, and `name` the import itself.

Thus, we want to normalize the path and make sure Windows paths use the
appropriate separator.

The name, however, is always given with posix style paths.