Previously we initialized trap handling (signals/etc) once-per-instance
but that's a bit too granular since we only need to do this as
one-time per-program initialization. This moves the initialization to
`Store` instead which means that we'll call this at least once per
thread, which some platforms may need (none currently do, they all only
need per-program initialization, but Fuchsia will need per-thread
initialization).
c284ffe6c0