Enable `curve25519-dalek/serde` #392

Job	Run time
cargo clippy	22s
Audit	12s
test (--features ristretto255-ciphersuite, --features serde, stable)	1m 20s
test (--features ristretto255-ciphersuite, stable)	57s
test (--features ristretto255-ciphersuite, 1.83.0)	1m 2s
test (--features ristretto255-ciphersuite, --features danger, stable)	1m 29s
test (--features serde, stable)	1m 16s
cargo fmt	17s
test (--features serde, 1.83.0)	1m 19s
test (--features ristretto255-ciphersuite, --features danger, 1.83.0)	1m 27s
test (1.83.0)	56s
test (stable)	1m 2s
test (--features ristretto255-ciphersuite, --features serde, 1.83.0)	1m 26s
Build with no-std on wasm32-unknown-unknown	20s
Build with no-std on wasm32-unknown-unknown	21s
Taplo	6s
test (--features danger, 1.83.0)	1m 16s
Build with no-std on thumbv6m-none-eabi	27s
Build with no-std on thumbv6m-none-eabi	13s
Build with no-std on wasm32-unknown-unknown	21s
Build with no-std on thumbv6m-none-eabi	13s
Build with no-std on wasm32-unknown-unknown	14s
Build with no-std on thumbv6m-none-eabi	18s
Build with no-std on wasm32-unknown-unknown	18s
Build with no-std on thumbv6m-none-eabi	13s
Build with no-std on wasm32-unknown-unknown	18s
Build with no-std on thumbv6m-none-eabi	13s
test (--features danger, stable)	1m 13s
	19m 9s

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