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

secret-memory/src/lib.rs

@@ -9,3 +9,6 @@ pub use crate::public::Public;
 
 mod secret;
 pub use crate::secret::Secret;
+
+pub mod policy;
+pub use crate::policy::*;