is instance `mutate.this` instated for type `mutate.this` and, if `mutate.this` does
not support use in several threads, check we are running in the thread `mutate.this`
was created in.

This is used a pre-condition for `exclusive` and `exclusive_when`.

Note that this is redefined in `blocking_mutate` since it does not require
exclusivity there, but it is sufficient if that is instated.