Add support for "switch" statements.

Switches with both uniform and varying "switch" expressions are
supported.  Switch statements with varying expressions and very
large numbers of labels may not perform well; some issues to be
filed shortly will track opportunities for improving these.

tests_errors/switch-4.ispc

@@ -0,0 +1,13 @@
+// "default" label illegal outside of "switch" statement
+
+void foo(float f) {
+  default:
+     ++f;
+    switch (f) {
+    case 1:
+        ++f;
+    case 2:
+        f = 0;
+    }
+}
+