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-3.ispc

@@ -0,0 +1,13 @@
+// "case" label illegal outside of "switch" statement
+
+void foo(float f) {
+    switch (f) {
+    case 1:
+        ++f;
+    case 2:
+        f = 0;
+    }
+ case 3:
+     --f;
+}
+