Choose task thread in rust_scheduler by round robin

[jamorton]

Remove the random context from rust_scheduler and use a simple round robin system to choose which thread a new task gets put on. Also, some incorrect tab indents around scoped blocks were fixed.

[brson]

This looks good. The only change I'd like is for cur_thread to be initialized to 0 in rust_scheduler's initialization list.

Also, I note that this will launch the first task on thread 1 instead of 0. Would make me happier if it started on 0, but not for any rational reason.

Thanks for fixing the indents. That was my mistake.

[brson]

Merged. Thanks!