Split part of the Task struct into a separate struct InCall
The idea is that this leaves Tasks and OSThread in one-to-one
correspondence. The part of a Task that represents a call into
Haskell from C is split into a separate struct InCall, pointed to by
the Task and the TSO bound to it. A given OSThread/Task thus always
uses the same mutex and condition variable, rather than getting a new
one for each callback. Conceptually it is simpler, although there are
more types and indirections in a few places now.
This improves callback performance by removing some of the locks that
we had to take when making in-calls. Now we also keep the current Task
in a thread-local variable if supported by the OS and gcc (currently
only Linux).