lang: funcs: core: fmt: Fix printf unification bug

We also add a backup fix to avoid a panic in case we ever hit a new
unification bug that lets something through, we can at least turn it
into a runtime issue. This adds a test as well.

lang/interpret_test/TestAstFunc2/printfunificationerr0.txtar

@@ -0,0 +1,5 @@
+-- main.mcl --
+import "fmt"
+test fmt.printf("%d%d", 42) {} # should not pass, missing second int
+-- OUTPUT --
+# err: errUnify: only recursive solutions left