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.

https://github.com/iqlusioninc/crates/blob/d86b82b172ef431af48cf5b8aee75ae9377a3033/secrecy/Cargo.toml#L25