Fuzz failure on master

[apoelstra]

All of the PRs merged in the last week have been merged without my testing because master is currently broken (fuzz tests fail). We should address this.

echo 1dVgWywL1VvV1dVAPQAAPTE= | base64 -d > hmm
cargo +nightly fuzz run display_parse_tree hmm

thread '<unnamed>' (2590207) panicked at fuzz/fuzz_targets/display_parse_tree.rs:9:10:
Output of fmt::Display should be parseable: RichError { error: Syntax { expected: ["something else"], label: Some("alias name"), found: Some("fn") }, span: Span { start: 5, end: 7 }, file: None }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Originally posted by @apoelstra in #213 (comment)