switch to atomic_shim in streamproducer

In order to support more platforms, especially MIPS, use atomic_shim::AtomicU64 for atomic operations, that transparently switches between native atomics and, for platforms that lack it, a mutex. Platforms like MIPS do not have native support for AtomicU64. This package depends on crossbeam-utils, which is already present in Cargo.lock, as a dependency for e.g. async packages.