Module sync

Some types from the std::sync module have ffi-safe equivalents in abi_stable::external_types.

The sync::{Mutex,RwLock,Once} equivalents are declared in abi_stable::external_types::parking_lot

The mpsc equivalents are declared in abi_stable::external_types::crossbeam_channel, this is enabled by default with the channels/crossbeam-channel cargo feature.