all: don't automatically convert userdata types to pointer types

One of the things I have done a better job of internalizing while
working with zig over the last few months is that the less magic that
exists, the better. In the case of parameterized functions, this means
that it is much better to restrict the range of types that are
permitted to be passed than to perform type manipulation. In other
words, it's more confusing to see a function that is parameterized
with `SomeType` taking a pointer to that type than having it be
parameterized directly to take the pointer. Obviously there are
exceptions to this rule, like std.mem.eql taking slices of its
parameterized type.

In fact, this new approach fixes some edge cases. Null userdata may now
be passed in, since the user can now actually specify an optional
pointer type (e.g. `?*void` may be used to provide always-null
userdata). Additionally, a pointer to a constant value can now be
passed in, which wasn't possible before (this could have been worked
around by use of constCast and being careful, but that's an
exceedingly bad option compared to having the type system work for
you).

tests/connection.zig

@@ -164,14 +164,14 @@ test "nats.ConnectionOptions" {
     try options.setMaxReconnect(10);
     try options.setReconnectWait(500);
     try options.setReconnectJitter(100, 200);
-    try options.setCustomReconnectDelay(u32, reconnectDelayHandler, &userdata);
+    try options.setCustomReconnectDelay(*u32, reconnectDelayHandler, &userdata);
     try options.setReconnectBufSize(1024);
     try options.setMaxPendingMessages(50);
-    try options.setErrorHandler(u32, errorHandler, &userdata);
-    try options.setClosedCallback(u32, connectionHandler, &userdata);
-    try options.setDisconnectedCallback(u32, connectionHandler, &userdata);
-    try options.setDiscoveredServersCallback(u32, connectionHandler, &userdata);
-    try options.setLameDuckModeCallback(u32, connectionHandler, &userdata);
+    try options.setErrorHandler(*u32, errorHandler, &userdata);
+    try options.setClosedCallback(*u32, connectionHandler, &userdata);
+    try options.setDisconnectedCallback(*u32, connectionHandler, &userdata);
+    try options.setDiscoveredServersCallback(*u32, connectionHandler, &userdata);
+    try options.setLameDuckModeCallback(*u32, connectionHandler, &userdata);
     try options.ignoreDiscoveredServers(true);
     try options.useGlobalMessageDelivery(false);
     try options.ipResolutionOrder(.ipv4_first);
@@ -179,8 +179,8 @@ test "nats.ConnectionOptions" {
     try options.useOldRequestStyle(false);
     try options.setFailRequestsOnDisconnect(true);
     try options.setNoEcho(true);
-    try options.setRetryOnFailedConnect(u32, connectionHandler, true, &userdata);
-    try options.setUserCredentialsCallbacks(u32, u32, jwtHandler, signatureHandler, &userdata, &userdata);
+    try options.setRetryOnFailedConnect(*u32, connectionHandler, true, &userdata);
+    try options.setUserCredentialsCallbacks(*u32, *u32, jwtHandler, signatureHandler, &userdata, &userdata);
     try options.setWriteDeadline(5);
     try options.disableNoResponders(true);
     try options.setCustomInboxPrefix("_FOOBOX");
@@ -200,7 +200,7 @@ test "nats.ConnectionOptions (crypto edition)" {
     defer options.destroy();
     var userdata: u32 = 0;
 
-    try options.setTokenHandler(u32, tokenHandler, &userdata);
+    try options.setTokenHandler(*u32, tokenHandler, &userdata);
     try options.setSecure(false);
     try options.setCertificatesChain(rsa_cert, rsa_key);
     try options.setCiphers("-ALL:HIGH");

tests/subscription.zig

@@ -113,7 +113,7 @@ test "nats.Subscription (async)" {
 
     {
         var count: u32 = 0;
-        const subscription = try connection.subscribe(u32, message_subject, onMessage, &count);
+        const subscription = try connection.subscribe(*u32, message_subject, onMessage, &count);
         defer subscription.destroy();
 
         const response = try connection.requestMessage(message, 1000);
@@ -126,7 +126,7 @@ test "nats.Subscription (async)" {
     {
         var count: u32 = 0;
         const subscription = try connection.subscribeTimeout(
-            u32,
+            *u32,
             message_subject,
             1000,
             onMessage,
@@ -144,7 +144,7 @@ test "nats.Subscription (async)" {
     {
         var count: u32 = 0;
         const subscription = try connection.queueSubscribe(
-            u32,
+            *u32,
             message_subject,
             "queuegroup",
             onMessage,
@@ -162,7 +162,7 @@ test "nats.Subscription (async)" {
     {
         var count: u32 = 0;
         const subscription = try connection.queueSubscribeTimeout(
-            u32,
+            *u32,
             message_subject,
             "queuegroup",
             1000,