Implemented DNS on blockchain. Beautified a lot of code, fixed some things.

src/dns/filter.rs

@@ -0,0 +1,16 @@
+use crate::dns::protocol::{QueryType, DnsPacket};
+
+pub trait DnsFilter {
+    fn lookup(&self, qname: &str, qtype: QueryType) -> Option<DnsPacket>;
+}
+
+pub struct DummyFilter {
+
+}
+
+#[allow(unused_variables)]
+impl DnsFilter for DummyFilter {
+    fn lookup(&self, qname: &str, qtype: QueryType) -> Option<DnsPacket> {
+        None
+    }
+}