Better explanations and extra functionality in `MutableBody` #7473

Job	Run time
perf	35m 54s
benchcomp-tests	3m 50s
documentation	2m 0s
regression (macos-13)	30m 21s
write-json-symtab-regression	15m 40s
regression (ubuntu-20.04)	23m 0s
regression (ubuntu-22.04)	24m 13s
regression (macos-14)	21m 8s
	2h 36m 6s

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