Secret memory with memfd_secret (#321)

Implements:
- An additional allocator to use memfd_secret(2) and guard pages using mmap(2), implemented in quininer/memsec#16
- An allocator that abstracts away underlying allocators, and uses specified allocator set by rosenpass_secret_memory::policy functions (or a function that sets rosenpass_secret_memory::alloc::ALLOC_INIT
- Updates to tests- integration, fuzz, bench: some tests use procspawn to spawn multiple processes with different allocator policies

rosenpass/tests/integration_test.rs

@@ -6,7 +6,7 @@ use std::{
     time::Duration,
 };
 
-use clap::{builder::Str, Parser};
+use clap::Parser;
 use rosenpass::{app_server::AppServerTestBuilder, cli::CliArgs};
 use rosenpass_secret_memory::{Public, Secret};
 use rosenpass_wireguard_broker::{WireguardBrokerMio, WG_KEY_LEN, WG_PEER_LEN};
@@ -275,6 +275,7 @@ fn check_exchange_under_dos() {
     fs::remove_dir_all(&tmpdir).unwrap();
 }
 
+#[allow(dead_code)]
 #[derive(Debug, Default)]
 struct MockBrokerInner {
     psk: Option<Secret<WG_KEY_LEN>>,