docs: Add block download example (#24)

examples/block-download/.gitignore

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+/target
+
+scratchpad
+.DS_Store

examples/block-download/Cargo.toml

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+[package]
+name = "block-download"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+pallas = { path = "../../pallas" }
+net2 = "0.2.37"
+env_logger = "0.9.0"
+hex = "0.4.3"
+
+[workspace]

examples/block-download/src/main.rs

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+use net2::TcpStreamExt;
+
+use pallas::ledger::alonzo::*;
+use pallas::ouroboros::network::blockfetch::{BatchClient, Observer};
+use pallas::ouroboros::network::handshake::{
+    n2n::{Client, VersionTable},
+    MAINNET_MAGIC,
+};
+use pallas::ouroboros::network::machines::primitives::Point;
+use pallas::ouroboros::network::machines::run_agent;
+use pallas::ouroboros::network::multiplexer::Multiplexer;
+use std::net::TcpStream;
+
+#[derive(Debug)]
+struct BlockPrinter;
+
+impl Observer for BlockPrinter {
+    fn on_block_received(&self, body: Vec<u8>) -> Result<(), Box<dyn std::error::Error>> {
+        println!("{}", hex::encode(&body));
+        println!("----------");
+        BlockWrapper::decode_fragment(&body[..])?;
+        Ok(())
+    }
+}
+
+fn main() {
+    env_logger::init();
+
+    let bearer = TcpStream::connect("relays-new.cardano-mainnet.iohk.io:3001").unwrap();
+    bearer.set_nodelay(true).unwrap();
+    bearer.set_keepalive_ms(Some(30_000u32)).unwrap();
+
+    let mut muxer = Multiplexer::setup(bearer, &vec![0, 3]).unwrap();
+
+    let mut hs_channel = muxer.use_channel(0);
+    let versions = VersionTable::v4_and_above(MAINNET_MAGIC);
+    let last = run_agent(Client::initial(versions), &mut hs_channel).unwrap();
+
+    let range = (
+        Point(
+            26249860,
+            hex::decode("915386f44ad3a7fccee949c9d3fe43f5a20459c7401f990e1cc7d52c10be1fd6")
+                .unwrap(),
+        ),
+        Point(
+            26250057,
+            hex::decode("5fec758c8aaff4a7683c27b075dc3984d8d982839cc56470a682d1411c9f8198")
+                .unwrap(),
+        ),
+    );
+
+    let mut bf_channel = muxer.use_channel(3);
+    let bf = BatchClient::initial(range, BlockPrinter {});
+    let bf_last = run_agent(bf, &mut bf_channel);
+    println!("{:?}", bf_last);
+}