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Don Cross c247250df7 C Windows: use GetSystemTimePreciseAsFileTime.
It turns out that GetSystemTimeAsFileTime only returns
time with millisecond resolution.
In order to get microsecond resolution in Astronomy_CurrentTime(),
I had to switch to GetSystemTimePreciseAsFileTime for Windows.

Example output from unit test:

C Test_AstroTime: PASS - realtime increment = 3.143e-07 seconds after 1 iterations.
2023-08-28 14:32:43 -04:00
..