Compute.ConcreteNew: add missing case for variant functions

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

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"}