Change serde features to allow no_std use

The `argh_shared` crate depends on `serde`, which uses `std` in its default configuration. This makes it incompatible with `no_std` crates.

As per the [`serde` docs](https://serde.rs/no-std.html), adding `default-features = false` makes it support `no_std`, and adding the `alloc` feature lets it opt back into memory allocation so it can still handle `String`s and such.

argh_shared/Cargo.toml

@@ -9,4 +9,4 @@ repository = "https://github.com/google/argh"
 readme = "README.md"
 
 [dependencies]
-serde = { version = "1", features = ["derive"] }
+serde = { version = "1", default-features = false, features = ["derive", "alloc"] }