Add support for multi-element vector swizzles. Issue #17.

This commit adds support for swizzles like "foo.zy" (if "foo" is,
for example, a float<3> type) as rvalues.  (Still need support for
swizzles as lvalues.)

tests/swizzle-4.ispc

@@ -0,0 +1,15 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_v(uniform float RET[]) {
+    float<3> a = {1,2,3};
+    float<2> b = a.xy - a.uv;
+
+    RET[programIndex] = b.x + 2*b.y;
+}
+
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}