Add cargo fuzz support

fuzz/.gitignore

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+target
+corpus
+artifacts
+coverage

fuzz/Cargo.toml

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+[package]
+name = "tiled-fuzz"
+version = "0.0.0"
+publish = false
+edition = "2018"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.4"
+
+[build-dependencies]
+glob = "0.3.1"
+
+[dependencies.tiled]
+path = ".."
+
+[[bin]]
+name = "tiled"
+path = "fuzz_targets/tiled.rs"
+test = false
+doc = false
+bench = false

fuzz/build.rs

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+use std::path::PathBuf;
+
+use glob::glob;
+
+macro_rules! p {
+    ($($tokens: tt)*) => {
+        println!("cargo:warning={}", format!($($tokens)*))
+    }
+}
+
+fn main() {
+    let path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    let corpus_dir = path.join("corpus/tiled");
+    std::fs::create_dir_all(&corpus_dir).expect("failed creating corpus dir");
+
+    let tmx_assets_path = path.join("../assets/*.tmx");
+    for entry in glob(tmx_assets_path.to_str().unwrap()).unwrap() {
+        match entry {
+            Ok(src) => {
+                let dest = corpus_dir.join(src.file_name().unwrap());
+                std::fs::copy(src, dest).unwrap();
+            }
+            Err(e) => {
+                p!("{:?}", e)
+            }
+        }
+    }
+}

fuzz/fuzz_targets/tiled.rs

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+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+use std::path::Path;
+
+use tiled::{DefaultResourceCache, Loader, ResourceReader};
+
+struct FuzzResourceReader<'a> {
+    data: &'a [u8],
+}
+
+impl<'a> FuzzResourceReader<'a> {
+    fn new(data: &'a [u8]) -> Self {
+        FuzzResourceReader { data }
+    }
+}
+
+impl<'a> ResourceReader for FuzzResourceReader<'a> {
+    type Resource = &'a [u8];
+    type Error = std::io::Error;
+
+    fn read_from(&mut self, _path: &Path) -> Result<Self::Resource, Self::Error> {
+        Ok(self.data)
+    }
+}
+
+fuzz_target!(|data: &[u8]| {
+    let mut loader =
+        Loader::with_cache_and_reader(DefaultResourceCache::new(), FuzzResourceReader::new(data));
+    let _ = loader.load_tmx_map("fuzz.tmx");
+});