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

Issue #183.

tests/memcpy-uniform.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    int32 * uniform src = uniform new int32[1024];
+    int32 * uniform dst = uniform new int32[1024];
+
+    foreach (i = 0 ... 1024)
+        src[i] = i;
+
+    memcpy(&dst[32], src, (1024-32)*sizeof(uniform int));
+    RET[programIndex] = dst[64+programIndex];
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 32 + programIndex;
+}