Extra tests for move and autoderef

[ijackson]

My now-closed-by-me MR #14 seemed simple and passed the in-tree if_chain tests but produced a number of regressions in projects of my own. That seems suboptimal. I have distilled the failure cases into these new tests, which pass on current if_chain and break with my #14.

[lambda-fairy]

Thanks for the tests!

I made a few simplifications which should help readability.

[lambda-fairy]

This can be simplified:

#[test]
fn magic_deref() {
    struct Wrapped<T>(T);
    let s = "hello";
    if_chain! {
        then { Wrapped(s) }
        else { Wrapped(&s) }
    };
}

[lambda-fairy]

This as well:

#[test]
fn no_move() {
    struct NoCopy;
    let x = NoCopy;
    if_chain! {
        then { drop(x); }
        else { drop(x); }
    }
}