New

StandardGpuResources can now be used by other indices through an immutable reference, thus allowing multiple GPU indices to share the same resources. The type is also Send now (but will never be Sync).
The new supertrait ConcurrentIndex contains methods for concurrent searching over an index. In this extended interface, search, range_search, and assign take &self instead of &mut self. FlatIndex implements ConcurrentIndex, more may come in the future.

Breaking Changes

renamed to_gpu and to_cpu methods to into_gpu and into_cpu, to comply with conventional Rust method naming.