Coroutine local variables

[zonyitoo]

Now in Coio, Coroutine is the minimum execution routine. It would be nice if we can have a coroutine_local! macro implementation just like thread_local! in std.

Implementation detail will be discussed later.

[zonyitoo]

Just to note:

1. If someone accesses to the variable without running Coroutine, it should panic or not? If not, what's the expected behavior?