This commit resumes management of the stack boundaries and limits when switching
between tasks. This additionally leverages the __morestack function to run code
on "stack overflow". The current behavior is to abort the process, but this is
probably not the best behavior in the long term (for deails, see the comment I
wrote up in the stack exhaustion routine).
Previously having optional lang_items caused an assertion failure at
compile-time, and then once that was fixed there was a segfault at runtime of
using a NULL crate-map (crates with no_std)
