Task local data management

Allows storing arbitrary types inside task-local-storage (TLS), to be accessed anywhere within a task, keyed by a global pointer parameterized over the type of the TLS slot. Useful for dynamic variables, singletons, and interfacing with foreign code with bad callback interfaces.

To declare a new key for storing local data of a particular type, use the local_data_key! macro. This macro will expand to a static item appropriately named and annotated. This name is then passed to the functions in this module to modify/read the slot specified by the key.

local_data_key!(key_int: int)
local_data_key!(key_vector: Vec<int>)

key_int.replace(Some(3));
assert_eq!(*key_int.get().unwrap(), 3);

key_vector.replace(Some(vec![4]));
assert_eq!(*key_vector.get().unwrap(), vec![4]);