Draft: Generic reduction function in GSFFindIndexOfMinimum

Pass a lambda to reduction function. Plan on using this in the future to absorb some other logic here.

Original:

6: ------------------------------------------------------------------------------------------------------------------
6: Benchmark                                                                        Time             CPU   Iterations
6: ------------------------------------------------------------------------------------------------------------------
6: benchmarkFindIdxOfMinimum<findIdxOfMinimum::Impl::C>                          6296 ns         6103 ns       114069
...
6: benchmarkFindIdxOfMinimum<findIdxOfMinimum::Impl::VecMinThenIdx>               465 ns          460 ns      1522466

Updated:

6: ------------------------------------------------------------------------------------------------------------------
6: Benchmark                                                                        Time             CPU   Iterations
6: ------------------------------------------------------------------------------------------------------------------
6: benchmarkFindIdxOfMinimum<findIdxOfMinimum::Impl::C>                          6297 ns         6096 ns       114708
...
6: benchmarkFindIdxOfMinimum<findIdxOfMinimum::Impl::VecMinThenIdx>               469 ns          460 ns      1519274