Add fuzzer that just parses &str input

fuzz/.gitignore

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+target
+corpus
+artifacts

fuzz/Cargo.toml

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+[package]
+name = "pulldown-cmark-fuzz"
+version = "0.0.0"
+authors = ["Automatically generated"]
+publish = false
+edition = "2018"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.4"
+
+[dependencies.pulldown-cmark]
+path = ".."
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[[bin]]
+name = "parse"
+path = "fuzz_targets/parse.rs"
+test = false
+doc = false

fuzz/fuzz_targets/parse.rs

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+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &str| {
+    for _ in pulldown_cmark::Parser::new(data) {};
+});