Implement, test, and document SHA256 #103

Job	Run time
posix (clang, clang++-12, 14,17,20, ubuntu-22.04, clang-12)	19m 29s
posix (clang, clang++-13, 14,17,20, ubuntu-22.04, clang-13)	19m 27s
posix (clang, clang++-14, 14,17,20, ubuntu-22.04, clang-14)	19m 25s
posix (clang, clang++-14, 14-gnu,17-gnu,20-gnu, ubuntu-22.04, clang-14)	19m 38s
posix (clang, clang++-15, 14,17,20, ubuntu-22.04, clang-15, deb http://apt.llvm.org/jammy/ llvm-t...	19m 28s
posix (clang, clang++-16, 14,17,20, ubuntu-22.04, clang-16, deb http://apt.llvm.org/jammy/ llvm-t...	19m 44s
posix (clang, clang++-17, 14,17,20, ubuntu-22.04, clang-17, deb http://apt.llvm.org/jammy/ llvm-t...	19m 45s
posix (clang, clang++-18, 14,17,20, ubuntu-24.04, clang-18, deb http://apt.llvm.org/noble/ llvm-t...	19m 47s
posix (clang, clang++-19, 14,17,20, ubuntu-24.04, clang-19, deb http://apt.llvm.org/noble/ llvm-t...	19m 45s
	2h 56m 28s

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