async memcpy

For #4

Add

• Implement Memcpy::copy_from_async using cuMemcpyAsync and tokio::task::spawn_blocking for awaiting cuStreamSynchronize
• impl Debug and PartialEq for PageLocakedMemory, DeviceMemory, and ResgiteredMemory for better debugging

Change

• Stream and Event now has ContextRef instead of Context !83 (merged)
• impl Copy for ContextRef

TODO

• async_trait for Memcpy
• for [T]
• for DeviceMemory, PageLockedMemroy and RegisteredMemroy
• for Array