add efficient user context manipulation routines (#1127) #403

Job	Run time
jammy/g++/c++17/asan=off/ubsan=off	7m 10s
focal/clang++/c++17/asan=off/ubsan=on	6m 16s
focal/g++-10/c++20/asan=on/ubsan=off	8m 5s
	21m 31s

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