Add fuzzer for decode functions (#115)

This commit adds a fuzzing harness to
fuzz the decode functions defined by
parity-scale-codec.

fuzzing/Cargo.toml

+[package]
+name = "codec_fuzzer"
+version = "0.1.0"
+authors = ["Vincent Ulitzsch <vincent@srlabs.de>"]
+edition = "2018"
+
+[dependencies]
+parity-scale-codec = { path = "../", features=["derive"]}
+honggfuzz = ""
+
+[workspace]

fuzzing/README.md

+# Codec fuzzer
+## Requirements: 
+Install [honggfuzz-rs](https://github.com/rust-fuzz/honggfuzz-rs):
+```
+cargo install honggfuzz
+```
+Run the fuzzer like so:
+```
+cargo hfuzz run codec_fuzzer
+```
+
+
+

fuzzing/src/main.rs

+use parity_scale_codec::{Encode, Decode};
+use std::collections::{BTreeMap, BTreeSet, VecDeque, LinkedList, BinaryHeap};
+#[cfg(not(fuzzing))]
+use std::io::{self, Read};
+
+#[derive(Encode, Decode, Default, Clone, PartialEq)]
+#[derive(Debug)]
+pub struct MockStruct{
+   vec_u: Vec<u8>
+}
+
+fn fuzz_one_input(data: &[u8]){
+    match data[0] % 9 {
+        0 => { let _u_16 = u16::decode(&mut &data[1..]);},
+        1 => { let _vec_u8= Vec::<u8>::decode(&mut &data[1..]);},
+        2 => { let _vec_u32 = Vec::<u32>::decode(&mut &data[1..]);}
+        3 => { let _linked_list = LinkedList::<u8>::decode(&mut &data[1..]);},
+        4 => { let _btree = BTreeMap::<u8, u8>::decode(&mut &data[1..]);},
+        5 => { let _btreeset = BTreeSet::<u8>::decode(&mut &data[1..]);},
+        6 => { let _vecdeque = VecDeque::<u8>::decode(&mut &data[1..]);},
+        7 => { let _binaryheap = BinaryHeap::<u8>::decode(&mut &data[1..]);},
+        8 => { let _mock_struct = MockStruct::decode(&mut &data[1..]);}
+        _ => unreachable!()
+    }
+}
+
+#[macro_use] extern crate honggfuzz;
+#[cfg(fuzzing)]
+fn main() {
+    loop {
+		fuzz!(|data: &[u8]| {
+            fuzz_one_input(data);
+	});
+	}
+}
+
+#[cfg(not(fuzzing))]
+fn main() -> io::Result<()> {
+    let mut buffer = String::new();
+    io::stdin().read_to_string(&mut buffer)?;
+    let data = buffer.as_bytes();
+    println!("Trying data: {:?}", data);
+    fuzz_one_input(data);
+    Ok(())
+}
+
+