Move NamedSharding.__eq__ and NamedSharding.__hash__ into C++.
* change NamedSharding::spec_ to use a more precise type, now PartitionSpec is a type known to C++.
* update PartitionSpec::Eq and PartitionSpec::Hash to be more usable from other C++ classes.
* add a thread-safe cache for the hash value of a NamedSharding.
* move the existing ShardingHash and ShardingEqual functions into jax_jit and rename them, since they are implementations that only really make sense for jit.
PiperOrigin-RevId: 770635344
