Improve performance by using Python C API to enter/exit context

[tarasko]

From perf, callstack before the change:

--54.22%--__pyx_f_6uvloop_4loop___uv_stream_on_read
         |          
          --52.82%--__pyx_f_6uvloop_4loop_run_in_context1
                    |          
                     --51.06%--cfunction_vectorcall_FASTCALL_KEYWORDS
                               context_run
                               |          
                                --50.49%--_PyObject_VectorcallTstate.lto_priv.18
                                          |          
                                           --50.28%--method_vectorcall
                                                     |          
                                                      --49.45%--__pyx_pw_6picows_6picows_10WSProtocol_13data_received

After the change:

--51.71%--__pyx_f_6uvloop_4loop___uv_stream_on_read
         |          
          --50.42%--__pyx_f_6uvloop_4loop_run_in_context1
                    |          
                     --49.13%--__pyx_pw_6picows_6picows_10WSProtocol_13data_received

On a side note, I have removed Py_INCREF/Py_DECREF because cython does it itself when method is called directly.

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 * cdef inline run_in_context(context, method):
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 *         return method()
 *     finally:
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  /*try:*/ {

    /* "uvloop/loop.pyx":97
 *     Context_Enter(context)
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