peepmatic: Define fuzz targets for various parts of peepmatic

fuzz/fuzz_targets/peepmatic_fst_differential.rs

@@ -0,0 +1,8 @@
+#![no_main]
+use libfuzzer_sys::fuzz_target;
+use peepmatic_fuzzing::automata::fst_differential;
+use std::collections::HashMap;
+
+fuzz_target!(|map: HashMap<Vec<u8>, u64>| {
+    fst_differential(map);
+});