revert back lifetime docs; make `execute` private #41

Job	Run time
debug (clang)	1m 43s
optimizations (clang, Release)	55s
style_check	12s
debug (gcc)	1m 41s
optimizations (clang, MinSizeRel)	32s
optimizations (gcc, Release)	37s
optimizations (gcc, MinSizeRel)	24s
	6m 4s

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