Move semantics for cross-task operations

[eholk]

Channel operations (mainly sending) should have move semantics and only be allowed for unique things.

This also applies to arguments to spawned functions, and probably the spawned function itself.

[eholk]

As of 54be5b0, this should be translating mostly correctly. There are still issues with typechecking it though.

[eholk]

With Graydon's work on move-mode arguments, and moving the task/comm system into a library, we're pretty much there. There still needs to be more safety checking on spawn though.

[brson]

Let's make sure spawn takes a ~fn and that it's safe

[eholk]

Spawn currently uses move-mode, so kind checking should hopefully enforce that you're giving it a ~fn.

[brson]

ok. I believe ~fn is not currently enforced - need to investigate. Further considerations: all of the things inside the spawned closure need to be ~, and all the environment allocs need to be migrated to the new task and dropped when the new task ends.

[brson]

fn~ is covered by #568, so I'm going to call this done. I'm sure we'll revisit task semantics post-0.1.