Fix some issues with gc tables (#1138)

The important one is the bug in using the load vector; it turns out it
was mistyped, it is supposed to be an f64vector of 3 elements, and was
using a plain vector of two floats... 🤦‍♂️

I also added a test for gc tables; not that it was triggering the bug
(that was triggered in a much more complex program), but it is nice to
have and we can make it more elaborate in time.