Implement `DerefMut` for `Log<T>` #2111

Job	Run time
check no_std	50s
docs	30s
clippy	59s
features stable	2m 16s
miri --no-default-features	4m 54s
fmt	14s
features stable --all-targets	3m 25s
miri	4m 43s
features nightly	1m 36s
miri --all-features	6m 27s
test ubuntu-latest stable --no-default-features	1m 22s
features nightly --all-targets	2m 52s
test ubuntu-latest stable	1m 29s
wasm	38s
test ubuntu-latest nightly --no-default-features	50s
test ubuntu-latest nightly	52s
test ubuntu-latest 1.81 --no-default-features	49s
test ubuntu-latest 1.81	42s
test windows-latest stable --no-default-features	2m 23s
test windows-latest stable	2m 25s
test windows-latest nightly --no-default-features	1m 50s
test windows-latest nightly	1m 47s
test windows-latest 1.81 --no-default-features	1m 26s
test windows-latest 1.81	1m 20s
test ubuntu-latest 1.81 --features json	50s
test windows-latest 1.81 --features json	1m 24s
test ubuntu-latest nightly --all-features	1m 10s
test windows-latest nightly --all-features	2m 42s
	52m 45s

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