geo-types: Weaken serde/std and approx/std features

Previously, if I wanted only the `std` feature of geo_types, this would
enable the `serde` and `approx` features, too. Change these features to
be "weak dependency features", so they're only enabled (and their crates
pulled in) if the corresponding optional crate is already pulled in for
other reasons.

geo-types/Cargo.toml

@@ -12,7 +12,7 @@ edition = "2021"
 
 [features]
 default = ["std"]
-std = ["approx/std", "num-traits/std", "serde/std"]
+std = ["approx?/std", "num-traits/std", "serde?/std"]
 # Prefer `use-rstar` feature rather than enabling rstar directly.
 # rstar integration relies on the optional approx crate, but implicit features cannot yet enable other features.
 # See: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#namespaced-features