Labels should be minified

Currently labels are not minified:

Input:

```js
abc: for (var a = [1, 2, 3, 4], i = 0; i < a.length; ++i) {
continue abc
}
def: for (var a = [1, 2, 3, 4], i = 0; i < a.length; ++i) {
for (var j = i; j < a.length; ++j) {
if (i + j > 7) {
continue def
}
}
}
```

Current output:

```js
abc:for(var a=[1,2,3,4],i=0;i

Start by reproducing the JavaScript input from the issue and locating the minifier logic that handles labels and continue statements. Compare the generated output with the expected output; done means labels are shortened where needed and continue statements use the corresponding shortened label.

Written by the indexing model from the issue text.