Improve repr of greenlets

[jamadden]

There's some possibly useful information that could be included in the default repr:

• Is it dead?
• Is it current?
• Is it the main greenlet?
• What thread ID is it attached to? (There have been some recent changes to Python thread IDs, IIRC, that might make that more interesting).

[jamadden]

I wasn't able to determine a cheap way to get any thread ID that would be meaningful to a user without using additional storage. We could print the ID of the thread state dictionary, and users could associate greenlets that are in the same thread that way, but it wouldn't tell them anything about the Python level thread. I'm not sure if that's helpful or not, so in the absence of feedback I'm leaning towards putting it in.