Bump `dbl` to v0.4.0-rc.1; MSRV 1.81 #309

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	14s
minimal-versions	0s
test (1.81.0)	17s
test (stable)	18s
build (1.81.0, wasm32-unknown-unknown)	14s
build (stable, thumbv7em-none-eabi)	16s
build (stable, wasm32-unknown-unknown)	18s
	1m 37s

Provide feedback