.chain_mut_ref() is the mutable alternative to .chain_ref().
A use case example for this method is extraction of an optional value
from an Option<Container> value.
Some notes about the commits.
Exit code propagation commits:
* ```Reimplement unwrap()``` has the same old code from ```arc::unwrap``` ported to use modern atomic types and finally (it's considerably nicer this way)
* ```Add try_unwrap()``` has some new slightly-tricky (but pretty simple) concurrency primitive code
* ```Add KillHandle``` and ```Add kill::Death``` are the bulk of the logic.
Task killing commits:
* ```Implement KillHandle::kill() and friends```, ```Do a task-killed check```, and ```Add BlockedTask``` implement the killing logic;
* ```Change the HOF context switchers``` turns said logic on
Linked failure commits:
* ```Replace *rust_task ptrs``` adapts the taskgroup code to work for both runtimes
* ```Enable taskgroup code``` does what it says on the tin.
r? @brson
r? @brson `rustpkg build`, if executed in a package source directory inside
a workspace, will now build that package. By "inside a workspace"
I mean that the parent directory has to be called `src`, and rustpkg
will create a `build` directory in .. if there isn't already one.
Same goes for `rustpkg install` and `rustpkg clean`.
For the time being, `rustpkg build` (etc.) will still error out if
you run it inside a directory whose parent isn't called `src`.
I'm not sure whether or not it's desirable to have it do something
in a non-workspace directory.
`rustpkg build`, if executed in a package source directory inside
a workspace, will now build that package. By "inside a workspace"
I mean that the parent directory has to be called `src`, and rustpkg
will create a `build` directory in .. if there isn't already one.
Same goes for `rustpkg install` and `rustpkg clean`.
For the time being, `rustpkg build` (etc.) will still error out if
you run it inside a directory whose parent isn't called `src`.
I'm not sure whether or not it's desirable to have it do something
in a non-workspace directory.
