Add Codecov configuration #375

Job	Run time
careful	1m 0s
coverage	32s
doc	25s
miri (s390x-unknown-linux-gnu)	1m 0s
test (stable)	44s
rustfmt	15s
clippy	22s
build (stable, armv7a-none-eabi)	1m 22s
miri (x86_64-unknown-linux-gnu)	47s
build (1.85.0, armv7a-none-eabi)	1m 20s
build (1.85.0, thumbv7em-none-eabi)	1m 22s
build (stable, wasm32-unknown-unknown)	1m 20s
build (stable, thumbv7em-none-eabi)	1m 17s
build (1.85.0, wasm32-unknown-unknown)	1m 21s
test (1.85.0)	48s
	13m 55s

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