RFC: Add a hook for detecting task switches.
Certain libraries are configured using global or thread-local state
instead of passing handles to every function. CUDA, for example, has a
`cudaSetDevice` function that binds a device to the current thread for
all future API calls. This is at odds with Julia's task-based
concurrency, which presents an execution environment that's local to the
current task (e.g., in the case of CUDA, using a different device).
This PR adds a hook mechanism that can be used to detect task switches,
and synchronize Julia's task-local environment with the library's global
or thread-local state.
