Allow ReadableStream.write(to:) to take a WritableStream _protocol value_.
Allow ReadableStream.write(to:) to take a WritableStream _protocol value_.

As opposed to a generic TextOutputStream value.

Tried to add this unit test:

```swift
func testWriteTo() {
	let (writer1,reader1) = streams()
	var (writer2,reader2) = streams()

	writer1.write("something")
	writer1.close()
	writer2.write("h")
	reader1.write(to: &writer2)

	XCTAssertEqual(reader2.read(), "something")
}

but the first `writer2.write` causes an EXC_BAD_ACCESS exception here:

```swift
extension WritableStream {
	public func write(_ x: String) {
		filehandle.write(x, encoding: encoding)
	}

```

No idea why.

kareman/SwiftShell,kareman/SwiftShell,kareman/SwiftShell