Remove `FromFn` trait #86

Job	Run time
careful	46s
clippy	11s
doc	16s
build (1.65.0, armv7a-none-eabi)	2m 46s
build (1.65.0, thumbv7em-none-eabi)	2m 48s
miri (x86_64-unknown-linux-gnu)	31s
rustfmt	6s
build (1.65.0, wasm32-unknown-unknown)	2m 39s
test (1.65.0)	2m 29s
miri (s390x-unknown-linux-gnu)	42s
build (stable, armv7a-none-eabi)	18s
test (stable)	11s
build (stable, thumbv7em-none-eabi)	54s
build (stable, wasm32-unknown-unknown)	10s
	14m 47s

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