Add memcpy(), memmove() and memset() to the standard library.

Issue #183.

tests/memmove-varying.ispc

@@ -0,0 +1,19 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int32 *buf = new int32[1024];
+
+    for (uniform int i = 0; i < 1024; ++i)
+        buf[i] = programIndex * 10000 + i;
+
+    if (programIndex == 2)
+        memmove(buf, buf+programCount/2, programCount*sizeof(uniform int));
+
+    RET[programIndex] = buf[0];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 10000 * programIndex;
+    RET[2] = 10000 * 2 + programCount/2;
+}