Remove all instances of `for in`

[stevendesu]

Per #13, I discovered that for in loops seem to allocate memory for some reason. This leads to gradually worse performance as you use for in loops over and over.

For now, we'll simply replace every instance of:

for (var key in obj) {
    // ... (use obj[key])
}

With:

var keys = Object.keys(obj);
for (var i = 0; i < keys.length; i++) {
    if (obj.hasOwnProperty(key)) {
        // ... (use obj[keys[i]])
    }
}