fix: store scopes in a stack to push/pop them in a consistent way

[twada]

Parent scope does not match parent function scope on leaving class
since espower handles scopes only at function level.

const assert = require('assert');
class Dog {
    say() {
        return 'bow';
        // gap occurs here
    }
}
const d = new Dog();
assert(d.say());

refs: power-assert-js/power-assert#78
refs: power-assert-js/power-assert#79

[coveralls]

Coverage increased (+0.006%) to 97.756% when pulling 5e9373f on scope-stack into 9163439 on master.

[coveralls]

Coverage increased (+0.006%) to 97.756% when pulling 5e9373f on scope-stack into 9163439 on master.