connection: eliminate String method variants

After contemplating this for a little bit, there's no point in exposing
these separately from the plain variants. Even nats.c internally calls
its plain variants after calling `strlen` on the input. Zig benefits
from having nicer pointer types than C, so we get "string" handling
for free from the fact that the standard variants take slices anyway.

examples/request_reply.zig

@@ -15,7 +15,7 @@ fn onMessage(
     });
 
     if (message.getReply()) |reply| {
-        connection.publishString(reply, "salutations") catch @panic("HELP");
+        connection.publish(reply, "salutations") catch @panic("HELP");
     }
 
     userdata.* = true;
@@ -30,7 +30,7 @@ pub fn main() !void {
     defer subscription.destroy();
 
     while (!done) {
-        const reply = try connection.requestString("channel", "greetings", 1000);
+        const reply = try connection.request("channel", "greetings", 1000);
         defer reply.destroy();
 
         std.debug.print("Reply \"{s}\" got message: {s}\n", .{