fixup! time: Move/Port timers to C++/Pyx nogil

This amends commit b073f6df: clang was complaining that _Ticker/_Timer
were forward-declared as class, but later declared as struct:

    ./golang/time.h:107:1: warning: '_Ticker' defined as a struct here but previously declared as a class [-Wmismatched-tags]
    struct _Ticker : object {
    ^
    ./golang/time.h:77:1: note: did you mean struct here?
    class _Ticker;
    ^~~~~
    struct
    ./golang/time.h:140:1: warning: '_Timer' defined as a struct here but previously declared as a class [-Wmismatched-tags]
    struct _Timer : object {
    ^
    ./golang/time.h:78:1: note: did you mean struct here?
    class _Timer;
    ^~~~~
    struct

-> Fix it by using struct. Also used inline style inside refptr.