Use `i32` rather than `int` in the Rust code

C's `int` is 32-bits, but the Rust's `int` is (currently) pointer-sized, i.e. 64-bits on x86-64. This changes the Rust to using a matching integer type, giving much better performance:

    75699
    LANGUAGE Rust 2167
    75699
    LANGUAGE C 2220
    75699
    LANGUAGE C 2271
Merge pull request #1 from huonw/rust-32-bit-integers

Use `i32` rather than `int` in the Rust code