Add a fresh fuzz corpus for the parse tree. (#217)
This was generated using the same steps as outlined for the lexer:
```
mkdir /tmp/new_corpus
./bazel-bin/parser/parse_tree_fuzzer -jobs=N /tmp/new_corpus
```
After some time, I stopped the fuzzer and then minimized and merged the
corpus:
```
rm parser/fuzzer_corpus/empty
./bazel-bin/parser/parse_tree_fuzzer -merge=1 parser/fuzzer_corpus /tmp/new_corpus
```
I'll add this short version to documentation in another PR.
Chandler Carruth