auto merge of #8139 : brson/rust/rm-old-task-apis, r=pcwalton

auto merge of #8139 : brson/rust/rm-old-task-apis, r=pcwalton

This removes a bunch of options from the task builder interface that are irrelevant to the new scheduler and were generally unused anyway. It also bumps the stack size of new scheduler tasks so that there's enough room to run rustc and changes the interface to `Thread` to not implicitly join threads on destruction, but instead require an explicit, and mandatory, call to `join`.