Introduce fuzz tests for core (#844)

This PR introduces fuzz tests and address #592

core/fuzz/src/main.rs

@@ -0,0 +1,75 @@
+extern crate grin_core;
+extern crate grin_keychain;
+
+use std::path::Path;
+use std::fs::{self, File};
+use grin_core::ser;
+use grin_core::core::{Block, BlockHeader, CompactBlock, Transaction};
+use grin_core::core::target::Difficulty;
+
+use grin_core::core::build::{input, output, transaction_with_offset, with_fee};
+use grin_keychain::keychain::Keychain;
+
+fn main() {
+	generate("transaction_read", &tx()).unwrap();
+	generate("block_read", &block()).unwrap();
+	generate("compact_block_read", &compact_block()).unwrap();
+}
+
+fn generate<W: ser::Writeable>(target: &str, obj: W) -> Result<(), ser::Error> {
+	let dir_path = Path::new("corpus").join(target);
+	if !dir_path.is_dir() {
+		fs::create_dir(&dir_path).map_err(|e| ser::Error::IOErr(e))?;
+	}
+
+	let pattern_path = dir_path.join("pattern");
+	if !pattern_path.exists() {
+		let mut file = File::create(&pattern_path).map_err(|e| ser::Error::IOErr(e))?;
+		ser::serialize(&mut file, &obj)
+	} else {
+		Ok(())
+	}
+}
+
+fn block() -> Block {
+	let keychain = Keychain::from_random_seed().unwrap();
+	let key_id = keychain.derive_key_id(1).unwrap();
+
+	let mut tx1 = tx();
+	let mut tx2 = tx();
+
+	Block::new(
+		&BlockHeader::default(),
+		vec![&mut tx1, &mut tx2],
+		&keychain,
+		&key_id,
+		Difficulty::one(),
+	).unwrap()
+}
+
+fn compact_block() -> CompactBlock {
+	CompactBlock {
+		header: BlockHeader::default(),
+		nonce: 1,
+		out_full: vec![],
+		kern_full: vec![],
+		kern_ids: vec![],
+	}
+}
+
+fn tx() -> Transaction {
+	let keychain = Keychain::from_random_seed().unwrap();
+	let key_id1 = keychain.derive_key_id(1).unwrap();
+	let key_id2 = keychain.derive_key_id(2).unwrap();
+	let key_id3 = keychain.derive_key_id(3).unwrap();
+
+	transaction_with_offset(
+		vec![
+			input(10, key_id1),
+			input(11, key_id2),
+			output(19, key_id3),
+			with_fee(2),
+		],
+		&keychain,
+	).unwrap()
+}