Better explanations and extra functionality in `MutableBody` #7483

Job	Run time
regression (macos-13)	26m 34s
benchcomp-tests	3m 39s
regression (ubuntu-20.04)	22m 55s
regression (ubuntu-22.04)	24m 31s
write-json-symtab-regression	15m 10s
regression (macos-14)	17m 2s
perf	35m 53s
documentation	2m 1s
	2h 27m 45s

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