bpfvm: A cBPF 'assembler' and virtual machine

bpfvm is a small BPF VM implementation and cBPF token 'assembler'. It is intended for testing cBPF functionality before deployment, e.g. seccomp BPF filters.

Example

```rust // Simple BPF opcode list //

[test_log::test]

fn testalumask() { let prog = vec![ bpfld(Mode::ABS, 2*WORDS), bpfstmt(libc::BPFALU | libc::BPFAND | libc::BPFK, 0xF0), bpfret(Src::Acc, 0), ]; let mut vm = BpfVM::new(&prog).unwrap();

let data = vec![0, 0, 0xFF, 0];
let ret = vm.run(&data).unwrap();
assert!(ret == 0xF0);

let data = vec![0, 0, 0x80, 0];
let ret = vm.run(&data).unwrap();
assert!(ret == 0x80);

}

// With tokens and assembler... //

[test_log::test]

fn testldgtret() { let asm = vec![ Load(Mode::IMM, 99), Jump(JmpOp::JGT, 98, Some("retacc"), Some("ret999")), // Should skip this one Label("ret999"), Load(Mode::IMM, 999), Label("ret_acc"), Return(Src::Acc, 0), ]; let prog = compile(&asm).unwrap();

let mut vm = BpfVM::new(&prog).unwrap();
let data = vec![];
let ret = vm.run(&data).unwrap();
assert!(ret == 99);

}

```