fmath.h: add span-based bit_unpack() utility (#4723) #7564

Job	Run time
(old) VP2021 gcc9/C++17 py3.7 exr3.1 ocio2.0	14m 0s
(old) VP2021 clang10/C++17 avx2 exr3.1 ocio2.0	14m 31s
(old) VP2022 gcc9/C++17 py39 exr3.1 ocio2.1	13m 3s
(old) VP2022 clang13/C++17 py39 avx2 exr3.1 ocio2.1	13m 21s
(old) oldest gcc9.3/C++17 py3.7 exr-3.1	13m 52s
(old) hobbled gcc9.3/C++17 py3.7 exr-3.1 no-sse	13m 51s
ABI check / c++17 py3.11	12m 54s
all local builds gcc12 C++17 avx2 exr3.2 ocio2.3 / g++-12 c++17 py3.10	18m 37s
bleeding edge gcc14 C++20 py3.12 OCIO/libtiff/exr-main avx2 / g++-14 c++20 py3.12	20m 34s
clang15 C++17 avx2 exr3.1 ocio2.2 / clang++-15 c++17 py3.10	14m 44s
clang-format / c++17 py3.10	33s
debug gcc9/C++17, sse4.2, exr3.1 / g++-9 c++17 py3.10	20m 17s
latest releases gcc13 C++20 py3.12 avx2 exr3.3 ocio2.4 / g++-13 c++20 py3.12	16m 47s
static libs gcc9 C++17 exr3.1 / g++-9 c++17 py3.10	13m 2s
VFX2023 gcc11/C++17 py3.10 exr3.1 ocio2.2 / c++17 py3.10	11m 57s
VFX2023 icx/C++17 py3.10 exr3.1 ocio2.2 qt5.15 / icpx c++17 py3.10	14m 41s
VFX2023 icc/C++17 py3.10 exr3.1 ocio2.1 qt5.15 / c++17 py3.10	19m 46s
VFX2024 clang/C++17 py3.11 exr3.2 ocio2.3 / clang++ c++17 py3.11	12m 13s
VFX2024 gcc11/C++17 py3.11 exr3.2 ocio2.3 / c++17 py3.11	13m 0s
VFX2024 sanitizers / clang++ c++17 py3.11	51m 21s
MacOS-13-Intel aclang14/C++17/py3.11 / clang++ c++17 py3.11	29m 35s
MacOS-14-ARM aclang15/C++20/py3.12 / clang++ c++20 py3.12	9m 50s
MacOS-15-ARM aclang16/C++20/py3.13 / clang++ c++20 py3.13	10m 10s
Windows-2019 VS2019 / c++17 py3.9	51m 3s
Windows-2022 VS2022 / c++17 py3.9	37m 4s
	7h 40m 46s

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