Add basic example for parsing the ancient demos with and without entities

This was mainly done to both start having some examples and try out the API.
They also serve as a starting point for benchmarking

examples/ancient-noentity.rs

@@ -0,0 +1,12 @@
+const DATA: &[u8] = include_bytes!("../testfiles/de_ancient.dem");
+
+fn main() {
+    let container = csdemo::Container::parse(DATA).unwrap();
+
+    let output = csdemo::parser::parse(csdemo::FrameIterator::parse(container.inner), csdemo::parser::EntityFilter::disabled()).unwrap();
+
+    println!("Header: {:?}", output.header);
+    println!("Players: {:?}", output.player_info);
+    println!("Events: {:?}", output.events.len());
+    println!("Entities: {:?}", output.entity_states.len());
+}