timer_wrap: remove HandleScopes, check return size

Calls from JS to C++ have an implicit HandleScope. So there is no need
to instantiate a new HandleScope in these basic cases.

Check if the returned int64_t is an SMI and cast the return value to
uint32_t instead of a double. Prevents needing to box the return value,
and saves a small amount of execution time.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>