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

Issue #183.

tests/memset-varying.ispc

@@ -0,0 +1,21 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int32 * varying buf = varying new int32[1024*(programIndex+1)];
+
+    if (programIndex & 1) {
+        memset(buf, 0xff, 1024*(programIndex+1)*sizeof(uniform int32));
+    }
+    else {
+        memset(buf, 0x01, 1024*(programIndex+1)*sizeof(uniform int32));
+    }
+
+    int v = buf[0];
+    int expected = (programIndex & 1) ? 0xffffffff : 0x01010101;
+    RET[programIndex] = (v == expected);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}