Support serialization of fixed-size arrays in abi_serializer.

[greg7mdp]

It is not clear we want to do this. It would require support in abios as well, in addition to abi_serializer, and verify that it works with cdt and other tools.

[bhazzard]

Ultimately we will adopt a stricter serialization approach.

As a first step, let's merely determine whether this was introduced in a recent CDT version (antelope era). If so, we'll reconvene to discuss next steps. If not, we'll likely icebox this, at least until after 6.0