bpo-21302: Add _PyTime_AsNanoseconds()

[vstinner]

Refactor pytime.c:

• Add pytime_from_nanoseconds() and pytime_as_nanoseconds(),
  and use explicitly these functions
• Add two empty lines between functions
• PEP 7: add braces { ... }
• C99: declare variables where they are set
• Rename private functions to lowercase
• Rename error_time_t_overflow() to pytime_time_t_overflow()
• Rename win_perf_counter_frequency() to py_win_perf_counter_frequency()
• py_get_monotonic_clock(): add an assertion to detect overflow when
  mach_absolute_time() unsigned uint64_t is casted to _PyTime_t
  (signed int64_t).

_testcapi: use _PyTime_FromNanoseconds().

https://bugs.python.org/issue21302