We implemented table.grow with an initializer argument before there was a spec for it, and the draft spec now takes the arguments in the opposite order of what we have, to follow
memory.fill: we should pop the delta first, then the init value, ie in the high-level notation, the init value is the first argument and the delta the second. This corresponds to the last two arguments of the fill instructions, which are initializer value and length.
Fixing this in the code is easy; but we have to update a number of test cases too, as well as the documentation on https://github.com/lars-t-hansen/moz-gc-experiments/.
We don't have to rev the gc_feature_opt_in version since that no longer applies to the reftypes features, but some people may experience a little pain if they've started to use the feature.