Bump `dbl` to v0.4.0-rc.1; MSRV 1.81 #319

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	15s
build (1.81.0, wasm32-unknown-unknown)	17s
test (1.81.0)	24s
build (stable, thumbv7em-none-eabi)	18s
test (stable)	19s
build (stable, wasm32-unknown-unknown)	19s
minimal-versions / minimal-versions	35s
	2m 27s

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