rustpkg: Make rustpkg commands work without a package ID

`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.
std::rt: Use a constant 4 threads for multithreaded sched tests. #7772

Too much overcommit here exhausts the low fd limit on OS X.
A major refactoring that changes the way the runtime uses TLS. In the
old design the TLS held the scheduler struct, and the scheduler struct
held the active task. This posed all sorts of weird problems due to
how we wanted to use the contents of TLS. The cleaner approach is to
leave the active task in TLS and have the task hold the scheduler. To
make this work out the scheduler has to run inside a regular task, and
then once that is the case the context switching code is massively
simplified, as instead of three possible paths there is only one. The
logical flow is also easier to follow, as the scheduler struct acts
somewhat like a "token" indicating what is active.

These changes also necessitated changing a large number of runtime
tests, and rewriting most of the runtime testing helpers.

Polish level is "low", as I will very soon start on more scheduler
changes that will require wiping the polish off. That being said there
should be sufficient comments around anything complex to make this
entirely respectable as a standalone commit.