Add fuzzing (#36)

* Add fuzzing

Fuzzing can be useful to find crashes on random input.
Running this for a short while should already result in a crash, proving
it's usefulness.

fuzz/.gitignore

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+target
+corpus
+artifacts

fuzz/Cargo.toml

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+
+[package]
+authors = ["Automatically generated"]
+edition = "2018"
+name = "grass-fuzz"
+publish = false
+version = "0.0.0"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+libfuzzer-sys = "0.3"
+
+[dependencies.grass]
+path = ".."
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[[bin]]
+doc = false
+name = "from_string_parsing"
+path = "fuzz_targets/from_string_parsing.rs"
+test = false

fuzz/README.md

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+# Fuzz
+Fuzzing targets for the grass library.
+
+## Installing
+You'll need `cargo-fuzz` for this to work, simply do:
+```
+cargo install cargo-fuzz
+```
+
+## Running
+Get a list of available targets with:
+```
+cargo fuzz list
+```
+
+And run a available target simply with:
+```
+cargo fuzz run <target>
+```
+You might have to use nightly:
+```
+cargo +nightly fuzz run <target>
+```
+
+
+
+## More info about fuzzing
+Consult the [fuzzing book](https://rust-fuzz.github.io/book/introduction.html).

fuzz/fuzz_targets/from_string_parsing.rs

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+#![no_main]
+use libfuzzer_sys::fuzz_target;
+
+
+fuzz_target!(|data: &[u8]| {
+    if let Ok(s) = std::str::from_utf8(data) {
+        let options = grass::Options::default();
+
+        let _ = grass::from_string(
+            s.to_owned(),
+            &options
+        );
+    }
+});