Don't use Enumerator to implement list Partitioner

...because it means the lists have to be realized on one thread only. Otherwise
we get a FiberError.
Remove unneeded CAS operations from Stream#realize

The subsequent mutex.synchronize operation provides a memory barrier anyways.
Rename Stream to LazyList

The name makes it immediately obvious what it is. "Stream" isn't so obvious.
Make List#partition thread-safe

The previous implementation was thread-safe FOR one list or the other, but NOT if
both of them were used by different threads at the same time.
Tweak docstring for LazyList
Rename Sequence to Cons

The name "Sequence" is too general; it could refer to almost anything.
Use Markdown in LazyList docstring
Add warning about multithreading to docstring for Hamster.enumerate