lang: funcs: simple: Check for variant signatures

This adds a safety check in case someone sneaks in a variant type in the
simple function signature. These might be sneaky to detect, and it's
simpler to catch them right here.

From a design point of view, we might consider actually permitting
these, like we did with the simple poly API, but it's probably better
for them to get implemented in that API instead (if we decide to allow
this long-term) and keep this simple API very simple.

lang/funcs/simple/simple.go

@@ -48,6 +48,10 @@ func Register(name string, fn *types.FuncValue) {
 	if fn.T == nil {
 		panic(fmt.Sprintf("simple func %s contains a nil type signature", name))
 	}
+	if fn.T.HasVariant() {
+		panic(fmt.Sprintf("simple func %s contains a variant type signature", name))
+	}
+
 	RegisteredFuncs[name] = fn // store a copy for ourselves
 
 	// register a copy in the main function database