Module native

Thread local support for platforms with native TLS.

To achieve the best performance, we choose from four different types for the TLS variable, depending on the method of initialization used (const or lazy) and the drop requirements of the stored type:

For const initialization and !Drop types, we simply use T directly, but for other situations, we implement a state machine to handle initialization of the variable and its destructor and destruction. Upon accessing the TLS variable, the current state is compared:

1. If the state is Initial, initialize the storage, transition the state to Alive and (if applicable) register the destructor, and return a reference to the value.
2. If the state is Alive, initialization was previously completed, so return a reference to the value.
3. If the state is Destroyed, the destructor has been run already, so return None.

The TLS destructor sets the state to Destroyed and drops the current value.

To simplify the code, we make LazyStorage generic over the destroyed state and use the ! type (never type) as type parameter for !Drop types. This eliminates the Destroyed state for these values, which can allow more niche optimizations to occur for the State enum. For Drop types, () is used.