Add cargo-fuzz target for DynamicAvx2Searcher

Add fuzzing to the project via cargo-fuzz and a target to test DynamicAvx2Searcher.

fuzz/.gitignore

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+target
+corpus
+artifacts
+coverage

fuzz/Cargo.toml

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+[package]
+name = "sliceslice-fuzz"
+version = "0.0.0"
+publish = false
+edition = "2021"
+
+[package.metadata]
+cargo-fuzz = true
+
+[dependencies]
+arbitrary = { version = "1.3.0", features = ["derive"] }
+libfuzzer-sys = "0.4"
+sliceslice = { version = "0.4.1", path = ".." }
+
+# Prevent this from interfering with workspaces
+[workspace]
+members = ["."]
+
+[profile.release]
+debug = 1
+
+[[bin]]
+name = "dynamic_avx2"
+path = "fuzz_targets/dynamic_avx2.rs"
+test = false
+doc = false

fuzz/fuzz_targets/dynamic_avx2.rs

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+#![no_main]
+
+use arbitrary::Arbitrary;
+use libfuzzer_sys::fuzz_target;
+use sliceslice::x86::DynamicAvx2Searcher;
+
+#[derive(Arbitrary, Debug)]
+struct FuzzInput<'a> {
+    needle: &'a [u8],
+    haystack: &'a [u8],
+    position: usize,
+}
+
+fuzz_target!(|input: FuzzInput<'_>| {
+    let mut input = input;
+
+    // This is a documented panic, so avoid it
+    if !input.needle.is_empty() {
+        input.position %= input.needle.len();
+    }
+
+    let result = unsafe {
+        let searcher = DynamicAvx2Searcher::with_position(input.needle, input.position);
+        searcher.search_in(input.haystack)
+    };
+
+    let expected = match input.needle.len() {
+        0 => true,
+        len => input.haystack.windows(len).any(|w| w == input.needle),
+    };
+
+    assert_eq!(result, expected);
+});