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).

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