context: Move/Port context package to C++/Pyx nogil
Provide context-related functionality that can be used directly from C++ and Pyx/nogil codes. Python-level classes and functions become small wrappers around pyx/nogil ones. Like with timers (b073f6df "time: Move/Port timers to C++/Pyx nogil") and interfaces (5a99b769 "libgolang: Start providing interfaces") memory for objects dynamically allocated on heap is managed automatically.
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