Add a rust user-space Capability struct.

This is a thin wrapper around a capability ptr that releases the
capability when it is done and prevents copying/cloning it.

To get a copy a caller must explicitly use duplicate.

rust/lib/mammoth/src/lib.rs

@@ -8,6 +8,8 @@ extern crate alloc;
 #[macro_use]
 pub mod macros;
 
+pub mod cap;
+mod cap_syscall;
 pub mod elf;
 pub mod init;
 pub mod mem;