Modified lock_api/Cargo.toml so that serde has no dependencies, not even default ones.
Modified lock_api/Cargo.toml so that serde has no dependencies, not even default ones.

@Amanieu pointed out that the `lock_api` crate needs to work in `no_std`
environments; using any dependencies in serde (including the default ones) would
result in the `lock_api` being unusable in `no_std` environments.  This change
should fix it so that it can be used in `no_std` environments.

Amanieu/parking_lot