Looks interesting PoC. I have written multiple extensions for PostgreSQL in Go. The hardest part is not "glueing" together the tools to make it work. Most of the time it's much more easier and faster to just write in C (C++ with wrapper).
There are many cases you should consider, like returning rows (composite types) and implementing trigger functions.
I disagree that it is easier or faster to just write it in C. But C still makes a lot more sense for many other reasons. And yes, this demos only the very basics, nothing fancy.
Anyways, a nice feature of Swift in this context is that Swift can directly access C APIs w/o any "wrapping". So while PL/Swift can add convenience APIs (like the ones it has to convert String's and Int's into Datum's), it can already do almost everything C can do due to that.
There are many cases you should consider, like returning rows (composite types) and implementing trigger functions.