This commit changes "ProcState" to store "file_fetcher" field in an "Arc",
allowing it to be preserved between restarts and thus keeping the state
alive between the restarts. File watchers for "deno test" and "deno bench"
now reset "ProcState" between restarts.
Turns out we were cloning permissions which after prompting were discarded,
so the state of permissions was never preserved. To handle that we need to store
all permissions behind "Arc<Mutex<>>" (because there are situations where we
need to send them to other thread).
Testing and benching code still uses "Permissions" in most places - it's undesirable
to share the same permission set between various test/bench files - otherwise
granting or revoking permissions in one file would influence behavior of other test
files.
