Change "use-serde" feature to "serde"

[tcharding]

We should the package re-name trick that we used in rust-bitcoin to rename the "use-serde" feature to plain "serde".

From rust-bitocin/Cargo.toml:

actual-serde = { package = "serde", version = "1", default-features = false, features = [ "derive", "alloc" ], optional = true }