This also improves test coverage for downloadToStreamByName().

Classes using CollectionWrapper should rely on its public API rather than work with its files and chunks collections directly.

Negative reads should throw before they cause an fread() failure. Zero-length reads can return an empty string immediately.

The Bucket's public API remains unchanged. More logic has been moved into the CollectionWrapper and the stream classes have also been renamed.

The StreamWrapper's registration has been changed to allow a configurable protocol (Bucket defaults to "gridfs"). While file paths are generated for readable and writable streams (now in a more robust manner), necessary arguments are passed through context options. We may decide to simplify the path for readable streams down the line, unless the current path is beneficial for debugging.

Closes #167