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).
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