Better explanations and extra functionality in `MutableBody` #7480

Job	Run time
benchcomp-tests	4m 53s
documentation	2m 15s
perf	35m 24s
write-json-symtab-regression	14m 10s
regression (macos-13)	39m 17s
regression (ubuntu-20.04)	23m 32s
regression (ubuntu-22.04)	23m 48s
regression (macos-14)	18m 31s
	2h 41m 50s

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