Move trap handler initialization to per-Store (#1644)

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).

crates/runtime/src/traphandlers.rs

@@ -253,7 +253,7 @@ cfg_if::cfg_if! {
 /// function needs to be called at the end of the startup process, after other
 /// handlers have been installed. This function can thus be called multiple
 /// times, having no effect after the first call.
-pub fn init() {
+pub fn init_traps() {
     static INIT: Once = Once::new();
     INIT.call_once(real_init);
 }