fix: use std::size_t as channel index type in nth_channel_view #545

Job	Run time
windows (msvc-14.2, 14,17, 32,64, windows-2019)	33m 35s
posix (gcc-6, 14,1z, ubuntu-latest, ubuntu:18.04, g++-6)	13m 37s
windows (gcc, 14,17, 64, windows-2019)	1m 37s
posix (gcc-7, 14,17, g++-7, ubuntu-20.04)	11m 58s
windows (msvc-14.3, 14,17, 32,64, windows-2022)	35m 27s
posix (gcc-8, 14, ubuntu-20.04, g++-8)	6m 24s
posix (gcc-9, 14,17, ubuntu-20.04, g++-9)	14m 52s
posix (gcc-10, 14,17, ubuntu-20.04, g++-10)	12m 50s
posix (clang, clang++-3.9, 14, ubuntu-latest, ubuntu:18.04, clang-3.9)	7m 45s
posix (clang, clang++-4.0, 14, ubuntu-latest, ubuntu:18.04, clang-4.0)	7m 35s
posix (clang, clang++-5.0, 14,1z, ubuntu-latest, ubuntu:18.04, clang-5.0)	13m 15s
posix (clang, clang++-6.0, 14,17, ubuntu-20.04, clang-6.0)	13m 28s
posix (clang, clang++-7, 14,17, ubuntu-20.04, clang-7)	13m 19s
posix (clang, clang++-8, 14,17, ubuntu-20.04, clang-8)	13m 30s
posix (clang, clang++-9, 14,17, ubuntu-20.04, clang-9)	13m 45s
posix (clang, clang++-10, 14,17, ubuntu-20.04)	14m 11s
posix (clang, 14,17, macos-11)	22m 24s
	4h 9m 32s

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