Add `Point::serialized_len` #366

Job	Run time
check	7s
build-wasm-nostd	28s
build-wasm	28s
check-all	19s
check-alloc	8s
check-changelog	1m 0s
check-doc	17s
check-semver (generic-ec, all-features)	25s
check-nostd	13s
check-semver (generic-ec, only-explicit-features)	22s
clippy	10s
check-semver (generic-ec-core, all-features)	54s
check-semver (generic-ec-core, only-explicit-features)	23s
clippy-nostd	17s
fmt	7s
check-semver (generic-ec-curves, all-features)	52s
clippy-tests	7s
check-semver (generic-ec-curves, only-explicit-features)	15s
check-semver (generic-ec-zkp, all-features)	48s
test	49s
check-semver (generic-ec-zkp, only-explicit-features)	22s
	8m 51s

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