Add trivial fuzzer for parser

As described in #61, fuzz testing some parts of this would be ~~fun~~
helpful. So, I started with the most trivial fuzzer I could think of:
Put random stuff into File::parse and see what happens.

To speed things up, I also did

    cp src/**/*.rs fuzz/corpus/parser/

in the `crates/libsyntax2/` directory (running the fuzzer once will
generate the necessary directories).

crates/libsyntax2/fuzz/fuzz_targets/parser.rs

@@ -0,0 +1,12 @@
+#![no_main]
+#[macro_use] extern crate libfuzzer_sys;
+extern crate libsyntax2;
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok(text) = std::str::from_utf8(data) {
+        let x = libsyntax2::File::parse(text);
+        let _ = x.ast();
+        let _ = x.syntax();
+        let _ = x.errors();
+    }
+});