update mutable field using a function of the old value

Different kinds of mutate will make different efforts for synchronization:

- for a thread-local `mutate`, this can be used only by the thread that instated
the `mutate`, no synchronization will be performed.

- for a `concur.blocking_mutate`/`concur.blocking_mutate_using_clock`, this will
use `exclusive` to ensure atomicity

- for a `atomic_mutate`, this will use a loop that performs `compare_and_swap`
repeatedly until the update was successful.

pre
  safety: open
  safety: !is_atomic || atomic_access_supported T