Add assignment operator

[tertsdiepraam]

For now, this can only assign to local variables and not to full paths, which is what I ultimately want to achieve as a (partial) solution to #156 and other issues.

Closes #159

Having an assignment operator is also useful for things like setting values based on if-else:

let x = 0;
if something {
    x = 3;
} else {
    x = 5;
}

You can write this as:

let x = if something { 3 } else { 5 };

But that gets annoying with multiple variables or when the branches call other functions.

Finally, this will make it useful to implement loops. Without assignment, it's quite hard to make loops useful.

---

The other use of this is with overriding default values:

type Config {
    do_foo: bool,
    do_bar: bool,
}

function default_config() -> Config {
    Config {
        do_foo: false,
        do_bar: false,
    }
}

fn setup() -> Config {
    let config = default_config();
    config.do_foo = true;
    config
}

[tertsdiepraam]

I think there's mostly 1 case not working properly right now:

x = x

This is hard because we both need to copy and drop x. The rest of the tests seem to pass valgrind.

I'm now pondering what the right solution is. The most proper thing to do would be to have some lifetime analysis which recognizes that the old x won't be used anymore and can therefore be moved instead of copied.

[tertsdiepraam]

I think that's all for this feature? It feels like such a big deal that it's hard to believe, but looks fine so far, even under Valgrind.

[bal-e]

No logic issues, just asking to move some control flow around.