lang: unification, interfaces: Don't pass over generators

We were skipping over being fully consistent with all of the generator
invariants when running the solver. This allowed us to miss some of the
conditions that a generator might impose. Usually this caused us to be
"solved" when in fact we had an invalid program.

lang/interpret_test/TestAstFunc2/printfempty.txtar

@@ -0,0 +1,10 @@
+-- main.mcl --
+import "fmt"
+
+# This should not unify, we need at least one arg.
+# NOTE: We have historically needed to turn on the recursive solver to find that
+# this was a bug!
+test fmt.printf() {}
+
+-- OUTPUT --
+# err: errUnify: 2 unconsumed generators

lang/interpret_test/TestAstFunc2/printfunificationerr0.txtar

@@ -2,4 +2,4 @@
 import "fmt"
 test fmt.printf("%d%d", 42) {} # should not pass, missing second int
 -- OUTPUT --
-# err: errUnify: only recursive solutions left
+# err: errUnify: 2 unconsumed generators