Bump `dbl` to v0.4.0-rc.1; MSRV 1.81 #120

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	19s
minimal-versions	0s
build (1.81.0, wasm32-unknown-unknown)	14s
test (1.72.0)	21s
build (stable, thumbv7em-none-eabi)	15s
test (stable)	18s
build (stable, wasm32-unknown-unknown)	16s
	1m 43s

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