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

Issue #183.

tests/memmove-uniform.ispc

@@ -0,0 +1,16 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int32 * uniform buf = uniform new int32[1024];
+
+    foreach (i = 0 ... 1024)
+        buf[i] = i;
+
+    memmove(&buf[1], buf, (1024-1)*sizeof(uniform int));
+    RET[programIndex] = buf[programIndex];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = max(0, programIndex-1);
+}