Support to pass unicorn handle to rust through FFI

[bet4it]

After this PR, we can do something likes this:
C code:

#include <unicorn/unicorn.h>
#include "rust_ffi.h"

int main()
{
	uc_engine *uc;
	int val = 0;
	uc_open(UC_ARCH_ARM, UC_MODE_ARM, &uc);
	rust_ffi(uc);
	uc_reg_read(uc, UC_ARM_REG_R0, &val);
	printf("R0: %d\n", val);
	uc_close(uc);
	return 0;
}

Rust code:

#![allow(non_camel_case_types)]

use std::ffi::c_void;
use std::convert::TryFrom;
pub type uc_handle = *mut c_void;
use unicorn_engine::Unicorn;
use unicorn_engine::RegisterARM;

#[no_mangle]
pub extern "C" fn rust_ffi(handle: uc_handle) {
    if let Ok(mut unicorn) = Unicorn::try_from(handle) {
        assert_eq!(unicorn.reg_write(RegisterARM::R0, 1234), Ok(()));
    } else {
        panic!("Failed convert handle to Unicorn")
    }
}

I think this PR is only meaningful after #1511 is merged, otherwise the memory layout of uc_engine may be different between Rust and C.

[bet4it]

@domenukk What do you think of it?

[domenukk]

Makes sense. Although I'd call the fn from_uc_ptr or from_handle and it needs to be declared unsafe (as you can shoot yourself in the foot).

You can also consider exposing the handle so you can do the inverse, calling a c fn with uc handle from rust.

Plus maybe make the fn take a void ptr? Just a thought

[bet4it]

Makes sense. Although I'd call the fn from_uc_ptr or from_handle and it needs to be declared unsafe (as you can shoot yourself in the foot).

I rewrite it with the TryFrom trait now. But I don't know how to declare unsafe on try_from😅

You can also consider exposing the handle so you can do the inverse, calling a c fn with uc handle from rust.

Done

Plus maybe make the fn take a void ptr? Just a thought

uc_handle and void ptr is totally the same thing:

unicorn/bindings/rust/src/ffi.rs

Line 10 in 5f89981

pub type uc_handle = *mut c_void;