rust: thread: Add Thread wrapper

[fbq]

It's working (on my machine), but sure, a lot to improve.

Updates (v9):

• Fix the potential memory leak found by @wedsonaf .
• Now we have two thread structure: RawThread and Thread<ThreadArg>, each thread struct has its own try_new function. try_new_closure is now kernel::thread::spawn.
• Other minor fixes.

Updates (v8):

• Rebase to the latest rust/rust.

Updates (v7):

• Rebase to the latest rust/rust to adopt sampe code changes.

Updates (v6):

• Fix style and typo as suggestions from @ojeda
• Add the case where a DST is shared between threads in examples, where try_new_closure() is slightly more appropriate than try_new().
• Fix a bug: the ThreadArg parameter of try_new() should have 'static lifetime, because the new thread may live longer.

Updates (v5):

• Make ThreadArg more easily to merge with PointerWrapper
• Take the suggestion from @ojeda on the function names (I still keep the try_ prefixes, as that indicates the return type is KernelResult not Thread
• Add the documentation section of "Memory Cost" for try_new_closure, so that the users will know the current drawbacks.

Updates (v4):

• Don't use macro to generate bridge functions
• Still surface try_new_c_style.

Updates (v3):

• Multiple document and style fixes
• Renames Thread::new to Thread::try_new because as the return type suggests the creation may fail.
• Adds Thread::try_new_c_style to allow people use C function pointer for thread creation.
• Moves the name parameter as the first parameter of Thread::try_new

Update:

• Documentation is added ;-)
• Thread::stop is added, to support waiting for a thread to finish.
• Correctly handle the refcount of the task_struct. during Thread::new and Thread::drop.
• Now Thread::new accepts FnOnce() -> KernelResult<()> as the closure for kernel threads.