secrecy: Using the serde feature in a no-std environment

[robin-nitrokey]

Currently, secrecy’s serde feature cannot be used in a no-std environment because it activates serde’s std feature. It would be useful if we could activate the serde feature without also activating std.

crates/secrecy/Cargo.toml

Line 25 in d86b82b

serde = { version = "1", optional = true }