Explicit Binding Scopes

This step makes the end of variable scopes explicit using the Explicit End Of Scope feature.

At the end of each scope, for each variable x declared in that scope in reverse order of declaration, we add a scope_end!(x) statement.

Before every break;/continue; statement, we similarly scope_end all the in-scope variables that will no longer be in scope at the target of the break;/continue;.

Finally before a return $local; statement we end the scopes of all locals except $local.

For example:

#![allow(unused)]
fn main() {
let x;
loop {
    let b;
    b = foo();
    let c;
    c = bar();
    if b {
        break;
    } else if c {
        return b;
    }
}

// becomes
let x;
loop {
    let b;
    b = foo();
    let c;
    c = bar();
    if b {
        scope_end!(c);
        scope_end!(b);
        break;
    } else if c {
        scope_end!(c);
        scope_end!(x);
        return b;
    }
}
scope_end!(x);
}