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perf: compute latLngToVec3 with math.Sincos in x/h3go (#156)

latLngToVec3 took the sine and cosine of both the latitude and the longitude
through four separate math.Sin / math.Cos calls, range-reducing each angle
twice. math.Sincos returns sine and cosine from a single reduction, so the
forward projection now makes two calls instead of four.

The projected output is unchanged: the pinned known-value tests, the cgo parity
suite and the conformance suite all still pass, and the path stays
allocation-free. latLngToVec3 is unexported, so unlike the other benchmarks it
cannot be reached from paritytest; BenchmarkLatLngToVec3 is added in-package to
guard it.

benchstat, fifteen interleaved rounds of each side on an Apple M3 Max:

                │     old      │                new                 │
                │    sec/op    │   sec/op     vs base               │
LatLngToVec3-16   13.750n ± 1%   8.614n ± 4%  -37.35% (p=0.000 n=15)

End to end the win is small, around 1% off LatLngToCell, since the projection is
well under a tenth of that call, which is dominated by the face-IJK descent.

Co-authored-by: gitKashish <125540624+gitKashish@users.noreply.github.com>

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