Support Cargo's "weak dependency features"

[dtolnay]

Reference:

• https://rust-lang.github.io/rfcs/3143-cargo-weak-namespaced-features.html
• rust-lang/cargo#8832

Example:

[dependencies]
serde = { version = "1.0", optional = true, default-features = false }

[features]
std = ["serde?/std"]