feat(fuzzing): Add initial set of fuzzing targets

These targets can be used with rust nightly and cargo-fuzz to fuzz
several bits of Rosenpass's API. Fuzzing is an automated way of
exploring code paths that may not be hit in unit tests or normal
operation. For example the `handle_msg` target exposed the DoS condition
fixed in 0.2.1.

The other targets focus on the FFI with libsodium and liboqs.

Co-authored-by: Karolin Varner <karo@cupdev.net>

fuzzing/fuzz_targets/kyber_encaps.rs

@@ -0,0 +1,19 @@
+#![no_main]
+extern crate arbitrary;
+extern crate rosenpass;
+
+use libfuzzer_sys::fuzz_target;
+
+use rosenpass::pqkem::{EphemeralKEM, KEM};
+
+#[derive(arbitrary::Arbitrary, Debug)]
+pub struct Input {
+    pub pk: [u8; 800],
+}
+
+fuzz_target!(|input: Input| {
+    let mut ciphertext = [0u8; 768];
+    let mut shared_secret = [0u8; 32];
+
+    EphemeralKEM::encaps(&mut shared_secret, &mut ciphertext, &input.pk).unwrap();
+});