afl: add main file

for testing that crashes have been resolved

previously, binary of the main crate was used, but targets may have more
validation than simply checking for panics

tests/afl/src/main.rs

@@ -0,0 +1,17 @@
+use std::io::Read;
+
+fn main() {
+    let mut args = std::env::args();
+    let _program = args.next();
+    let target = args.next().expect("no target");
+    assert_eq!(args.next(), None);
+
+    let f = match target.as_str() {
+        "parse" => jotdown_afl::parse,
+        _ => panic!("unknown target '{}'", target),
+    };
+
+    let mut input = Vec::new();
+    std::io::stdin().read_to_end(&mut input).unwrap();
+    f(&input);
+}