Add cargo-fuzz target

fuzz/fuzz_targets/fuzz_target_1.rs

@@ -0,0 +1,7 @@
+#![no_main]
+#[macro_use] extern crate libfuzzer_sys;
+extern crate simple_asn1;
+
+fuzz_target!(|data: &[u8]| {
+    let _ = simple_asn1::from_der(data);
+});