feat: add preliminary miri support

- Adds a devShell with Miri
- Marks some of the tests which Miri cannot execute as ignored for Miri

Signed-off-by: wucke13 <wucke13+github@gmail.com>

rp/src/key.rs

@@ -128,6 +128,7 @@ mod tests {
     use crate::key::{genkey, pubkey, WG_B64_LEN};
 
     #[test]
+    #[cfg_attr(miri, ignore)] // Miri does not support calls to mmap with protections other than PROT_READ|PROT_WRITE
     fn test_key_loopback() {
         secret_policy_try_use_memfd_secrets();
         let private_keys_dir = tempdir().unwrap();

rp/tests/smoketest.rs

@@ -2,6 +2,7 @@ use std::process::Command;
 
 #[cfg(any(target_os = "linux", target_os = "freebsd"))]
 #[test]
+#[cfg_attr(miri, ignore)] // unsupported operation: extern static `pidfd_spawnp` is not supported by Miri
 fn smoketest() -> anyhow::Result<()> {
     let tmpdir = tempfile::tempdir()?;