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-1.ispc

@@ -0,0 +1,9 @@
+// Case statement value must be a compile-time integer constant
+
+void foo(float f) {
+    switch (f) {
+    case 1.5:
+        ++f;
+    }
+}
+