Expose OSThreadID and assorted functions from Haskell

Currently given two ThreadIds, there is no way to tell without FFI if they are running on the same OS thread. There is not even a way to ask what OS thread we are currently on: something like myThreadId except myOsThreadId.

Currently to remedy this, the user has to effectively copy and paste parts of the internal rts files and use something like .hsc with CPP to get at this information. This is not really portable on the user side and requires nasty gutting.

In the RTS the function we are interested in is at least osThreadId: with this function we can inspect current OSThreadId and communicate it out through an MVar or otherwise.

So this is a feature request for an abstract type OSThreadId with instances of at least Eq, Show and a function myOsThreadId :: IO OSThreadId.

Nice to have would be osThreadId :: ThreadID -> IO OSThreadId but not critical. If there is room for optimisation in GHC, functions like sameOsThread :: ThreadId -> ThreadId -> IO Bool would also be nice but again not critical.

Not sure what to set in ticket meta-data so please change as appropriate.