Compute.ConcreteNew: add missing case for variant functions

Also adding a test case in the test suite for this.

src/compiler/GF/Compile/Compute/ConcreteNew.hs

@@ -267,6 +267,7 @@ vapply v vs =
 --  VClosure env (Abs b x t) -> beta gr env b x t vs
     VAbs bt _ (Bind f) -> vbeta bt f vs
     VS (VV t fs) s -> VS (VV t [vapply f vs|f<-fs]) s
+    VFV fs -> vfv [vapply f vs|f<-fs]
     v -> bug $ "vapply "++show v++" "++show vs
 
 vbeta bt f (v:vs) =

testsuite/compiler/compute/Variants.gf

@@ -0,0 +1,7 @@
+resource Variants = {
+  oper
+    hello = r.f "hello";
+    r = { f:Str->Str = (id|dup) };
+    id : Str->Str = \ s -> s;
+    dup : Str->Str = \ s -> s++s;
+}

testsuite/compiler/compute/Variants.gfs

@@ -0,0 +1,2 @@
+i -retain testsuite/compiler/compute/Variants.gf
+cc hello

testsuite/compiler/compute/Variants.gfs.gold

@@ -0,0 +1 @@
+variants {"hello"; "hello" ++ "hello"}