Unsound Send/Sync bound for Device and DeviceHandle

[Qwaz]

Hello fellow Rustacean,
we (Rust group @sslab-gatech) found a memory-safety/soundness issue in this crate while scanning Rust code on crates.io for potential vulnerabilities.

Issue Description

rusb/src/device.rs

Lines 34 to 35 in 12ee91d

	unsafe impl<T: UsbContext> Send for Device<T> {}
	unsafe impl<T: UsbContext> Sync for Device<T> {}

rusb/src/device_handle.rs

Lines 127 to 128 in 12ee91d

	unsafe impl<T: UsbContext> Send for DeviceHandle<T> {}
	unsafe impl<T: UsbContext> Sync for DeviceHandle<T> {}

Device and DeviceHandle implement Send and Sync trait for all T types that implement UsbContext. UsbContext trait has neither Send nor Sync bound and can be implemented from the user side. This permits writing a custom non-thread safe UsbContext implementation in safe Rust code, which can cause a data race when used with Device or DeviceHandle.

If UsbContext is not expected to be implemented by users, making UsbContext a sealed trait can solve this problem. Otherwise, a proper bound should be added to Send/Sync implementations of Device and DeviceHandle (<T: Send/Sync + UsbContext>) or to the definition of UsbContext (UsbContext: Send + Sync).

[Qwaz]

Thanks for the fix!