Add fast path for ReadableStream.from(readableStream)

[MattiasBuelens]

ReadableStream.from() usually requires its argument to be an iterable or an async iterable. However, async iteration over a native ReadableStream is not yet well supported in browsers, so it cannot be used to convert a native stream to a polyfill stream.

This adds a fast path for converting anything that "looks like" a ReadableStream to a polyfill ReadableStream:

• The stream must have a getReader() method, which must return a default reader.
• The default reader must have read() and cancel() methods.

Fixes #1